A RAM upgrade may help
a little with Adobe products, but that's not the real
"cause for the slowdown".
The problem is you have a platter-based hard drive inside.
With any of the more recent versions of the Mac OS, that equates to = SLOW.
You have two choices to fix the problem:
1. Open it up and put an SSD inside. This can be a risky job unless you have the skill and confidence to do it.
or
2. Buy an EXTERNAL USB3 SSD, plug it in, and set it up to be the boot drive. This will yield about 85% of the speed the same drive would give you if it was installed internally. That's
very good, and "going external" is a fast and easy way to get A LOT MORE speed out of the iMac.
Something like this would do the job and do it right:
https://www.amazon.com/SanDisk-1TB-...keywords=sandisk+extreme+usb+3+portable+drive
Either a 1tb or 512gb would be fine, depending on your budget.
If you get a 1tb SSD, you could use CarbonCopyCloner to just "clone over" your internal OS install to the external SSD. CCC is FREE to download and use for the first 30 days, so this won't cost you. (but CCC is the best backup app there is, bar none, worth registering).