I was having problems with my 2016 iMac while it was still under Apple Care. My computer ran ok from a fresh boot but slowed waaaaaaaay down the longer I had the computer on. After trying a few other possible solutions the Apple tech recommended installing and running Dr Cleaner. It does restore performance back to fresh boot up levels.
As a side note about this service call, one of the first things the tech wanted to do was remote into my system so he could look at settings, programs running, etc. I told him I was ok with that but it probably wouldn’t work too well, as my internet speed wasn’t very good even as advertised and I seldom reached 1/4 of that speed. The opening dialogue/permission window took 4 or 5 minutes to open, I clicked on ‘ok’ for the permissions notice and then we waited. 5 minutes later he still had not connected, though from indications the program itself was running on my computer.
That’s when he recommended Dr Cleaner, and I spent a couple of hours downloading it. This is the reason I hate Apples move toward wireless and cloud storage while de-emphasizing downloading programs. Downloading takes a long time. Wireless or cloud computing never worked at all.
I since have had a speed upgrade, so it’s not as bad now, but you don’t have to travel very far from where I live to be back under those conditions. And other than satellite there are no other options for ISP’s.