slow iMac after big sur update

The best antivirus, cleaning app, and overall maintenance app for Mac OS is Mac OS itself. All you need to do with Mac OS to keep it secure and running well is to keep it up-to-date and do periodic (about 1x per week) restarts. Other than that, leave it alone. Adding third party antivirus, cleaning, security and other types of maintenance apps to Mac OS adds no additional level of security. The only thing these apps do is have the opposite affect users want. They make Mac OS slow, unstable, generate odd behavior (much like you are experiencing) and make Mac OS appear buggy. 

Please locate the developers uninstall instructions for Avast and follow to the letter. Then restart in Safe Mode per the directions in Use safe mode to isolate issues with your Mac and then restart normally. 

The best antivirus, cleaning app, and overall maintenance app for Mac OS is Mac OS itself. All you need to do with Mac OS to keep it secure and running well is to keep it up-to-date and do periodic (about 1x per week) restarts. Other than that, leave it alone. Adding third party antivirus, cleaning, security and other types of maintenance apps to Mac OS adds no additional level of security. The only thing these apps do is have the opposite affect users want. They make Mac OS slow, unstable, generate odd behavior (much like you are experiencing) and make Mac OS appear buggy. 

Please locate the developers uninstall instructions for Intego and follow to the letter. Then restart in Safe Mode per the directions in Use safe mode to isolate issues with your Mac and then restart normally. 

I would also recommend that you read https://www.apple.com/macos/security/ , in short Apple does not advocate the use of any third party antivirus, cleaning or any other third party maintenance apps because they are simply unnecessary. There are NO viruses in the wild for Mac OS, not one. There is other malware however users can easily avoid them and a third party antivirus app simply will not find them. I think you will find that almost all experienced users on these forums agree and advocate not using third party antivirus apps.

rkaufmann87 said it very well. Avast is definitely problematic. What happens is that an existing underlying problem will often manifest after an OS upgrade. It is not the fault of the upgrade. It is due to the existing issue and that is what must then be cleared. Therefore, the uninstall of Avast, which no Mac, ever, has any need for. You will see the difference.

I would like to add something you just did a major upgrade and Spotlight is indexing your system leave it on overnight and such and it should speed up after a while and when Spotlight finishes. I did so after my upgrade and after awhile it was back to normal. And it has done the same on previous Major Upgrades (Not minor or security updates).

jchapper, clearly anything I say is very likely to be discounted by you since you have very clearly stated your knowledge about using antivirus applications and you indicate that you feel that youare far more knowledgeable than some of the people above. (Whom I have a great deal of respect for based on their past postings here.) With all due respect I will, however, offer the following information for others who are reading these postings. The Apple operating system is now divided into two sections as represented in the Disk Utility app. The first is Macintosh HD and the second is Macintosh HD - Data (unless they are renamed). The Macintosh HD drive is read only, meaning that it is not possible to write to it or to delete any of it. It can not be infected by any virus. Absolutely can not. The second drive is where we install our apps and store files. Additionally, the Mac OS has its own security system built in. We generally will recommend the Malwarebytes app because it is safe and causes no harm. Technically it does not deal with viruses as such, only with Malware. Since it does not run in the background, constantly checking files, it does not slow down the computer at all. Given that a large percentage of complaints posted on this forum are about slowness and given that many of the problems are found to be caused by antivirus apps, and that the problem is resolved when such apps are removed, I believe the facts speak for themselves. I can certainly respect anyone's right to believe otherwise.

While anti-virus software can sometimes affect performance of a Mac to a certain degree, in particular while virus checks are being performed in the background, I would respectfully disagree with your statements that "Intego is an unnecessary antivirus app" and that "Macs do not need any antivirus protection."

Could you please back up your claims with real-world facts, such as benchmarks, statements from security experts that conclusively prove that Macs do not need antvirus protection and so on? Macs are NOT fully protected from viruses by default, any more than any other OS. Removing an anti-virus app may seem to improve performance in some ways, but you will also be exposed to other security vulnerabilities such as malware that targets Macs. I have used Intego products for many years now, and while I find that they do cause a significant performance hit while doing full scheduled virus checks in the background (which can be disabled), the real-time virus check feature does not cause any performance issues for me and never has. In fact, it has saved me from malware infection a few times.

The myth of macOS being secure and fully protected has been debunked over and over again. No platform is so robust and secure that it is fully protected against security threats on its own.

Also, your advice to upgrade past 16GB RAM is probably unnecessary for the OP, unless they are using heavily RAM-intensive apps, editing video or crunching massive amounts of data. If the computer worked great before the Big Sur upgrade and is now slow, then it follows that there must be either (1) an extra burden on the system caused by the OS upgrade (which I do not see on my Macs upgraded to Big Sur), or (2) some incompatibility between the OP's installed third-party apps and Big Sur. Those are the things to explore first, before advising the user to spend money on RAM. Granted, RAM is generally an easy and cheap upgrade (if the user's Mac will allow it), but the software side and incompatibilities are where they should look first.

I should also add that Intego Antivirus is experiencing some problems on Big Sur as of Jan 19 2021. Apps may be slow to load or may crash on first boot. This is an issue that myself and other users have reported. From this perspective, the OP might benefit by uninstalling Intego AV for the time being and going with a different reputable anti-virus product. However, I would not recommend going without anti-virus protection—macOS is not that robust or impervious to all security attacks, and it never was.

There are NO known self propagating viruses for Macs, none! There is, however, adware and malware which the user has to approve to install, often unwittingly or unknowingly. For that type malware download and run the free version of Malwarebytes. It was developed by a long time contributor to these forums and a highly respected member of the computer security community. 

As it's been said before, there is no reason to ever install or run any 3rd party "cleaning", "optimizing", "speed-up", anti-virus, VPN or security apps on your Mac.  This user tip describes what you need to know and do in order to protect your Mac: Effective defenses against malware and other threats - Apple Community   

I just unchecked the optimize battery recharging and that seems to help a bit.

By chance do you have any antivirus, cleaning or any other third party maintenance apps installed?

I have a similar problem. Before upgrading to Big Sur, my imac was running to my satisfaction. My imac is 2017 21.5inch 2.3GHz dual-core intel core i5 with 8GB memory and a 256GB SSD drive. After the upgrade my imac is performing at snail's pace.

If I try to run more than one app at a time, it is even slower.

For example, just starting the new Safari is about 5 times slower, just starting MS Word or Excel is about 20 time slower.

A program I am running now (maxXvideo) previously took about 10 mins to do a particular task - it has now been running for 75 mins and i still only about 70% completed.

Have I done something wrong? What do I have to do to get back to how things were before the update?

I have no antivirus and my Mac runs slow after the BigSur update. It seems to be getting slower each day. Even apps like Mac Mail are slow - when I move the cursor down to the next message in a folder, the color ball shows up and whirls for a few seconds before anything happens. What's going on?

Ronasara, thanks for your reply.

I must admit that I have been using third-party anti-virus apps for nearly all of the years that I have been computing on both Windows and macOS platforms. For Windows, the use of these apps was required by my employers. Even now as a freelancer, my customers and contracted employers also require the use of the same.

I have no "skin in the game" as far as this conversation goes, and I'm willing to accept that I might be wrong. I can understand (and even sympathize with) the general consensus that anti-virus apps are a waste on macOS, in that they often come with their own performance issues. I'd also say that I can understand when people's "slowness" problems "magically disappear" when such apps are removed, in some cases. I would rather not have to pay for the license or install one at all.

I also know about (and have used MalwareBytes on the Windows platform), and can agree with your recommendation. I'm not sure why you did not recommend that to the OP in the beginning (even though as you said, it is not an anti-virus app, but rather anti-malware), but you probably should, rather than leave them with the impression that they don't need anything and that all such software is useless.

The thing is, we often believe that something is "impossible" (such as a security breach) and then do not take all available measures to prevent it. If viruses were not a "thing" on the Mac, then why do such anti-virus apps even exist? Let's assume you're totally right as far as macOS goes, that there has never and will never be a virus that could possible exist or harm a Mac user. What about Mac users who deal with Windows files and users on that platform? Most good anti-virus apps will help quarantine and deal with Windows viruses. For Mac users that have to work with Windows users, such an app seems worthwhile, wouldn't you say?

We also know that macOS is not an impenetrable platform. Any platform and OS has attack vectors, and that's partially why vendors like Apple and Google pay handsome bounties to those who can find security breaches. Security holes are found all the time—the release notes for each version of macOS confirm this. Granted, the ones that are found are generally patched (we hope); but what about those that are not patched but found by someone else? I don't think we can place 100% of our trust on any single manufacturer when it comes to security. In fact, I think we need to behave as if anything can and will happen. Getting rid of our anti-virus software because it's "slow" may seem like an acceptable solution, but I think the better approach is to assume that anti-virus apps are actually designed to do something useful, and are provided for the benefit of the user's security rather than to detract from it.

Even things like the read-only partitions you mentioned work because the OS itself does not grant access. That said, do you remember the time when Apple made a huge security blunder (I believe this was back in High Sierra) and allowed root access without a password through the Preferences panel? True, this was patched relatively quickly... but it just underscores that security is not a given. Software and hardware are complex beasts. There may be hundreds of security issues right now that still remain unpatched, and exploits in the wild we can't even imagine.

It's assumed by us that we can trust the operating systems and software we work with. Probably, we can. I respect that you are trying to help users, and I'm sure you have a lot of experience doing so. But reading what you wrote, I just felt that you are advocating throwing out the baby with the bath water, and that you are asking users to put an unshakeable level of trust in their operating system without doing anything on their own part to make it more secure. That's at least how I felt, when I read what you said.

I offer the following in response to your questions above. You asked "What about Mac users who deal with Windows files and users on that platform?" As I understand it, this is not a problem as Windows viruses have no affect on the Mac's operating system since they are very different. I fully understand that some employers require their people to use antivirus apps, even if they have Macs. In these cases, I suggest that they can look into having the antivirus app installed, but turned off except when certain interactions dictate that be activated. That way, the overall performance is not affected. As to your other concerns, all I can suggest that we can help protect our operating systems by being very careful about what we download and where we download files from. The rest, I will leave to Apple to deal with. Finally, I respect that everyone has to make their own decisions in these matters.

I am running Avast antivirus but have not had any problems with that before.

You have a good computer with a faster solid state drive and 16 GB of RAM. That is acceptable, but you can add more RAM and I would definitely do so. You may also get some other suggestions from other people, but the first things you need to do remove Intego and App Cleaner Helper. Intego is an unnecessary antivirus app which runs in the background and will seriously slow down the computer. It may also cause operations problems, as App Cleaner Helper can do as well. Go to their respective web sites and download their uninstallers, then run them. Macs do not need any antivirus protection as the OS is fully protected already. These two removals should substantially improve your computer's performance.