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I upgraded to High Sierra yesterday.
Is this on the HDD or SSD?

The Finder has been sluggish, but it's getting better. I don't regret the choice.
Right after installing the OS, indexing takes place, and your boot drive might have a significant performance loss.

Indexing usually takes a few hours to a few days to fully complete. Performance will progressively improve as indexing progresses.
 
High Sierra is on the HDD. Still on HFS+. Didn't upgrade automatically to AFS, which is what I wanted. The drive is 1TB WDC WD10EZEX-00WN4A0

Recovery Mode worked, unlike in Sierra. Slow but it worked. I got the spell checker back in Pages and was able to upgrade 1Password for free. I do all searching with EasyFind. I usually leave Spotlight indexing turned off. I realized many foreign fonts were dumped onto my drive, so tonight I deleted them. That may have been causing the slow down. I've used Kurt Lang's site http://www.jklstudios.com/misc/osxfonts.html many times over the years.

Safari somehow got worse and more frustrating to use. I never use it. I've always used Firefox. Brave isn't bad, but I prefer Firefox. Well, I may have to use Safari to login to my router. I've had that problem before.
 
High Sierra is on the HDD. Still on HFS+. Didn't upgrade automatically to AFS, which is what I wanted. The drive is 1TB WDC WD10EZEX-00WN4A0

Recovery Mode worked, unlike in Sierra. Slow but it worked. I got the spell checker back in Pages and was able to upgrade 1Password for free. I do all searching with EasyFind. I usually leave Spotlight indexing turned off. I realized many foreign fonts were dumped onto my drive, so tonight I deleted them. That may have been causing the slow down. I've used Kurt Lang's site http://www.jklstudios.com/misc/osxfonts.html many times over the years.

Safari somehow got worse and more frustrating to use. I never use it. I've always used Firefox. Brave isn't bad, but I prefer Firefox. Well, I may have to use Safari to login to my router. I've had that problem before.

I heard that APSF doesn't go well with old HDDs.
On my iMacs, I could only format SSD to APFS. on the HDD, there are no option to choose APFS. Doesn't matter anyway, as I removed all the HDDs out of my iMacs to preserve the fans' life.
 
When I decided to try Mac, a friend had a 2011 iMac for sale. Not wanting to spend a lot of money, I bought it and then discovered the 4 gig ram was upgradable to 16. I then found a YT video showing how to replace the spinning HD with an SSD. So I did that and added four 4-gig memory sticks and reinstalled MacOS. Ran great for a couple of years until the wife and I both got MacBook Pros. At which time the iMac was retired.
 
I bought it and then discovered the 4 gig ram was upgradable to 16.
The max RAM was actually 32GB on the Mid 2011 iMacs, unless you had the Education edition (Late 2011) which has a max of 16GB.

For the Mid 2011 iMacs, 16GB was the official number from Apple because there was no (or not a lot) 8GB sticks. After the 8GB sticks came out, it was known that the Mid 2011s could go up to 32GB.

Apple never revises the original specs, so if you look up the max RAM from Apple for most Macs, it is often no correct.
 
Everything can be replaced.
I never noticed the fans...until I played GRID. Then the Mac sounds like a jet airplane. I know there's more than one fan in the Mac, but I don't know which one I'm hearing during GRID. All of them, maybe.
 
The 21.5 inch 2011 iMac has three fans. One on the right for the ODD and GPU, one in the middle for the HDD and one on the left for the CPU.
 
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