Buy New or Upgrade?

You can not upgrade the CPU and/or GPU in an all-in-one iMac.

Catalina 10.15 is the highest macOS that the 2012 iMac can run.

The good news is, that Catalina will continue to receive security update for a couple more years.

The bad news is, that if you want to run Big Sur, Monterey and beyond, then you will need a new/newer iMac.

➡️ First, your Security and Updates are disabled. Please go to System Preferences > Software Update, and turn on the following:

• • Check for Updates
  • Download new updates when available
  • Install App Updates from the App Store
  • Install System data files and security updates

Then, go to System Preferences > Security & Privacy > General, and make sure the "Allow Apps downloaded from" is set to either "App Store" or "App Store and Identified Developers" (that may be the only options shown).

➡️ Your external backup drive is too small, and it is poor quality. Consider upgrading to a larger backup drive, such as a LaCie 4TB Rugged USB-C + USB 3.0 Portable Hard Drive - Apple.

➡️ You are approaching low disk space.

➡️ Other than that, the reason your iMac is very slow is due to the 7200 rpm hard drive. If you use an external SSD and run macOS from that, it will make your Mac 10 to 30 times faster for storage. For more info, instructions, and what external SSD to buy, please read: How to Setup and Use an External SSD as y… - Apple Community.

➡️ "Vivaldi" is using a lot of RAM! Consider using Safari instead. It is far more efficient.

Cheers,

Jack

Chances are, there is an easy and cost-effective solution to dramatically increase performance (an external SSD). However, to check, I believe we need to proceed with an EtreCheck report. EtreCheck is a useful App to diagnose and identify issues, both hardware and software. It is trusted by many users. Please follow the steps outlined below:

1. Navigate to: https://etrecheck.com and get the free version of EtreCheckPro.
2. Download it. It is a free and trusted app. It is often recommended here on Apple Support Communities. 
3. Open EtreCheck, and before running the report, be sure to check “Allow full Drive Access” at the bottom of the EtreCheckPro window.
4. Run the report, and put it in a post, as seen here: https://discussions.apple.com/docs/DOC-250000211. We will then examine the report for issues in your Mac.

Jack

I love keeping old Macs on the firing line line but this one is a tough call. The 2012 iMac can run no higher macOS than 10.15 Catalina so that makes spending money questionable at this point, right before a new macOS version Monterrey is released.

I'm wondering if it's possible to install the latest and greatest processor (and more?) and thereby turn this computer into one that will operate like a brand-new one.

I do not see a processor change as a viable option due to cost, uncertainty of success, and the fact the your iMac case is sealed and a bear to work on. And, if the processor is NOT the issue but it turns out instead to be something much more common, you've accomplished nothing. Read on.

What is common? The majority of "Slow iMac" reports here are due to slow mechanical hard drives that were de rigueur in 2012. Unless the computer was ordered with a Fusion drives or a factory solid-state drive (SSD), data transfer speeds were usually under 200MB/sec. My 2017 iMac with factory SSD does 2700MB/sec. That alone will make the computer feel like it is frozen in syrup.

As it is best practice to see what is making your current computer slow so you can make informed decisions, we need performance data that show how both software and hardware issues are affecting your speed. Fortunately there is a safe, secure way to do that. 

We can quickly and within the confines of these forums help you determine what issues are at play if you use EtreCheck Pro, available here:

https://etrecheck.com/index

The free version will do nicely for this purpose, although the app is worthy of our financial support.

We can see hard data about drive performance, software issues, and RAM usage. Etrecheck is the development of a long-serving and trusted contributor here expressly for displaying information in these forums to help us help you. It will not reveal any personal or secure information.

Due to forum software limitations, you cannot directly paste the report into a reply here. See this excellent User Tip for how to add the text of the report without garbling the results or having the report truncated:

How to use the Add Text Feature When Post… - Apple Community

There are many reasons why a computer starts running slow. I would look into fixing that first. I recommend downloading, installing and running the free app EtreCheck. Be sure to enable full disk access. Then post the resultant report below under "Additional text". Someone here can then look at it and offer some possible fixes.

First your free space on your boot drive is inadequate. You should free up a minimum of 80-100 GB of space to give the system some space to breath and operate.

The files that you have control over are located in the Documents, Downloads, Pictures, Music and Movies folders.  You can use either of these two free apps, GrandPerspective  or OmniDiscSweeper, to find the largest files on your drive so you can determine if they can be deleted or moved to an external HD for storage.  

I suggest you get a 4 GB external HD for Time Machine. I recommend drives from OWC (MacSales.com) as they have proven reliability, excellent customer support and warranties. These would be excellent choices: OWC Mercury Elite Pro 7200 RPM Storage - self powered and inexpensive.

After copying your backups from your current TM drive to the new one erase your current Time Machine drive and move files from the boot drive to it. If you move a PHotos library to the EHD be sure to open it on the EHD to make sure it works before deleting from the boot drive. Same would go for your iTunes music library if you move it.

After fixing software issues you could run from an external SSD to speed things up. Clone your internal Mojave using Carbon Copy Cloner

I suggest that you give careful consideration to your needs before ordering. I would only order a computer with a solid state (SSD) hard drive. It's much faster. Keep in mind that most likely you will also have to upgrade many of your apps. Some will have an upgrade cost. These days, SSD external drives are reasonably priced, small and easy to connect. Enjoy.

This is what I did wrong. I bought an old iMac year 2015. Its much slower than my older iMac year 2010.

I thought that year 2015 is upgradable to Monterey so I'm future proof, silly me :(

Seems the hard disk is 5200 RPM which is making it super slow.

Older OSX (High Sierra) might work fine on it.

I've ordered an external Samsung SSD and HOPE that it'll become fast.

Till then I'm running Garuda Linux on it. Linux seems to be running just fine even on old hard disks.

So I would say, spend and get something which runs fast (SSD is not just good to have, its a MUST).

Don't be "me", spend more and get a decent enough hardware.

Something seems amiss with your backup drive. Please go to Disk Utility, and select "View" > "Show All Devices". Select each device (or entry shown) in the external section, and take a screenshot of it and upload them all here.