iMac slow after Catalina instal.

Late 2013 21.5" iMac, 8 GB Ram, 1TB HDD. After update to Catalina 10.15.1, the thing was turning into a brick. Nothing worked, Office 365 took 3-4 attempts to launch and only did so after 10 minutes. Chrome, Firefox, Safari were crashing continuously. Creative Cloud, Photoshop, Lightroom, a nightmare. Performance was slow and buggy across the board. Wanted to throw the thing out of the window but, being stubborn, decided that no machine and some crappy software were going to beat me. The age of Terminator ain't not quite here yet. Started in Safe Mode, suddenly everything worked, like a gazelle compared to a Soviet agricultural tractor of the 1950s. It wasn't a matter of CPU or Memory usage, which were all well within the green. Here's what worked for me:

In System Preferences:

Turned off Siri, since I don't use it.

Turned off font smoothing and dynamic desktop.

Removed most of the extensions that I wasn't using anyway, almost 90% of them.

Turned off Notifications for all the things that I didn't care to know about in real time.

Made sure that there were no unneeded Login Items under the User and Group preferences (which in my case were none).

Changed the DNS servers in my WiFi connection to Cloudflare's faster ones (1.1.1.1, 1.0.0.1 and their IPv6 equivalents).

This improved things somewhat, but it was still not performing as expected.

Then:

Removed all the Adobe and Creative Cloud processes that were running in the background and prevented them from launching at login in the first place by deleting all Adobe launchers in HD/Library/LaunchDaemons, HD/Library/LaunchAgents and usr/Library/LaunchAgents. That did it. Restart, and suddenly everything worked. The Verifying App dialog now takes 2-3 minutes instead of an hour, Office 365 launches in 20 seconds, all three browsers launch in seconds as well. Adobe apparently is not yet fully compatible with Catalina and just messes everything up in the background. Will use GIMP for the time being. If I have to use Photoshop or Lightroom for some reason, will have to do a quick file cleanup afterwards - like cleaning up after your dog. Now the birds are out, the sun shines, and I feel like a better human being. Hope this helps!

SOLVED!!

after the disinstallation of malwarebytes my ssd gained 400Mb/s in reading speed. now i have 2300 Mb/s reading and 1400 Mb/s writing. the ssd now has only 100Mb/s less in reading and writing speed compared to mojave.

I will never install app cleaners again...

Just wanted to share something I discovered that helped me. Maybe it will help someone else too.

I've been battling slow boot on both of my Mac's (Specs below), immediately after moving to Catalina. I tried reducing the number of startup applications, startup "kext" services (Not sure if this is the right term), clearing cache in various places, etc.

The solution that just worked for me was clearing off my desktop. I saw something on another thread that pointed this out, so figured it was worth a shot! I was trying anything at this point. I had several screenshots, old .dmg installers, random folders, and various other junk on both desktops. Moved these to a different folder elsewhere on my drive, and both Macs immediately started booting quicker. Both were taking a minute or more to boot up. It's now under 20 seconds, probably closer to 10-15 seconds.

Disclaimer: Of course, this isn't a 100% verified solution, but it helped both of my Macs. Definitely something Apple should address.

Computer Specs:

Mac #1

Type: Mac Mini

Model: Late 2012

Processor: 2.6 GHz Quad-Core i7

Memory: 16GB DDR3

MacOS Version: 10.15.2

Drive: 1TB Samsung 960 EVO SSD - 670+ GB free (Upgraded after Catalina install, actually. But, same performance as last drive)

Mac #2

Type: Macbook Pro

Model: 2017 - 13" two thunderbolt ports, no touchbar

Processor: 2.3 GHz Dual-Core i5

Memory: 8GB LPDDR3

MacOS Version: 10.15.2

Drive: 256GB PCIe Flash memory (Installed by Apple) - ~100GB free

Other things to note:

Both Macs have FileVault encryption on

Both Macs have iCloud Desktop sync turned on

Once again, not sure if this will help everyone, but it did help me! Quite significantly, actually.

I updated my mac os software from Catalina version ?? to Catalina, version 10.15.1. Afterwards, my internet connection was so slow that it was virtually unusable. After spending a day and various attempted fixes, I am happy to say that the issue has been corrected. My speed is now blazing fast! The fix, for me, was to remove my Trend Micro antivirus software. Supposedly, you are not to install an outside or third party Antivirus software on a mac. I am very proud of myself! Roland Ott Jr.

Same for me. As dgoodin suggested, I signed out of iCloud and all is back to normal which is better than it has been in a long while. I even suffered this issue before Catalina. I hope Apple can fix this as I do like having iCloud synchronization across my families’ devices.

Well I have been having the same issues and finally fixed it. I tried all the suggestions, Spotlight on/off, clear Pram, making radical setup changes and nothing, still ran like a brick.

So I cloned the system from the internal hard drive onto and external StarTech.com hard drive running SSD via the lighting port and its back up to speed.

So basically, the older iMacs must have disc based hard drives and it just can't cope with it.

My thanks to @TheSubbornGuy. Both Apple and Adobe need to correct their respective products.

I did my own test: I shut down my Adobe Acrobat, and sure enough, the computer is very significantly improved. 

I have Adobe Acrobat Pro DC 2015 for Mac, because I use Acrobat for scanning into ".pdf".  This version of the product has always sucked, when compared to prior versions.  It is a prime example of a Alexander's principle which states that "a mature software product is degraded by each upgrade." (There are "Peter Principle" type reasons for this.) 

I have my own guess regarding the problematic relationship between Adobe products and Catalina OS. The fake solutions borne of echo-chamber ignorance usually point to memory or CPU or a need to do some kind of "clean up;" it's not that. My observations indicate that there has to be something going on with the handling of asynchronous process, or signals, and/or the scheduler, or the such. It has to be something that makes requests upon the OS, and that can then bottle-neck the OS, but doesn't use CPU nor memory.  Something is seizing low level, high priority access to the OS scheduler; which points to i.o. handlers. Catalina is allowing Adobe and other problem processes to displace everyone else's normal access to the scheduler; this is the OS equivalent of a "denial of service" attack.  During the long pauses, the cursor often disappears from the screen; note that the cursor is representative of the mouse, and the mouse is an i.o. device; this strongly suggests the problem has the low interrupt level and high priority of i.o. handling. One seemingly credible commenter said he switched to an SSD and got some relief; disk storage is another form of i.o.

At the practical level, my problem is that I use ".pdf" a lot. Every time I open an old invoice or other ".pdf" document, Acrobat runs. I guess I have to remove Acrobat from my computer (even though I paid for it) and find some free-ware alternative. And what about people who have come to depend on Adobe products, like Photoshop or InDesign, for earning their living; what are they supposed to do?

And why is Catalina allowing itself to be highjacked? A proper OS should protect itself from highjacking.

I wish I had never installed Catalina.

This issue is happening with all Antivirus software packages. I had to uninstall WebRoot and was going to install another application until I read the majority have the same issues. Your antivirus will run forever without actually doing anything. This is being reported by WebRoot, Trend Micro, and many others. It all started with Catalina being updated. Total waste!!!!!!!!

Have a look at the memory tab and see what is using memory. My 2015 iMac was running very slowly when RAM was all used because a Swap File on HDD was in use. In my case it was Music but ......

I'd also observe that since sorting the number of issues and using machine it seems to be speeding up further with time which make me wonder about indexing etc.

Sorry, cannot be much more help.

Late 2013 iMac ran well with Mojave - installed Catalina things started to slow when opening apps. Finally got to where screensaver using Photos album could not open (timed out). After four days of reading these posts and searching the web

(also this issue helped me clean out/move my docs and movies) found a fix:

Remove non Apple or non App Store apps, shut down iCloud Drive (leave iCloud for Photos on). Your stuff stays on iCloud Drive.

Reboot, restart iCloud Drive for all except Desktop and Document folders. (Go to iCloud Drive in the sidebar, create a new documents folder(which you'll be able to move) inside Documents folder(which you can't move) to hold stuff you need on your iMac.

Reinstall desired non Apple apps (in my case Adobe reader), and retrieve new documents folder from iCloud.

I also reloaded Malwarebytes but found it slowed the system response. Uninstalled it and rebooted.

Result: Boot and apps starts still slower than Mojave BUT in app/browser actions seem quicker/smoother.

My screensaver now finds its smart album (last 3 years) and shows our photos when I leave the Mac.

Interestingly no memory or CPU overload ever showed up while investigating why it's running slow.

Catalina is slower than Mojave to open stuff.

Reading 64 bits takes more time than 32 bits?

Thank you!! My 2013 iMac 27 has been very slow since the recent Catalina 10.15.3 update, pinwheels with almost everything. Just saw your fix and deleted my Avira anti-virus and its many related links. My iMac seems to be back to normal. Thanks again for the tip.

Removing Adobe solved the issue for me!!!!!! I went from a crippled computer back to normal funtioning nearly instantly. Thankful for TheStubbornGuys solution!!!!!!!!!!!

I had to uninstall all 32bit software and Microsoft Office and had to reinstall all Adobe softwares. It runs noticeably faster but still way slower than before Catalina

Your computer will continue to run slow while having your antivirus active. I know this sucks, but you are not alone right now with having that as your only option. You have no other choice but to shut it down. Mine would not allow me to do that and for some odd reason was saying I didn't have administrator rights to do it. I own the computer and I haven't changed any settings. You will have to go into safe mode by pressing the shift key after typing in your password to get it there. Then just drag your application to the waste bin to delete. Again, I know this sucks, but I couldn't get mine to stop its endless loop and allow me to utilize my computer. I'm not officially without until Apple works with other software providers. Apple......you are an Ahat! for doing this to your consumers.

Same for me, only it started before Catalina. For me, it's an iCloud problem. When completely signed out, the iMac screams; when signed in, I scream. Apple engineers are working on it.