That being said, it feels like it's slowing down. Has anyone had success reviving the performance of their older machine? Anyone tried backing up and doing a restore?
I upgraded everything in it I could though, CPU, RAM, screen, etc.
The biggest and best change I made to it was replacing the main HD with a nice SSD. Now it can run everything I need, except some games. I usually had 2-4 VMs running something throughout the work day.
I also took out the optical drive and replaced it with a OWC adapter and another SSD.
The last thing I do is back up everything I need, wipe and reinstall everything. I usually do this once a year or when needed, and it helps quite a bit with it feeling / acting slow.
(not sure if you can easily upgrade the hard drive or not though, but it made a world of difference for me)
That was the moment I decided to set up the systems fresh from scratch on a blank SSD (keeping the old one in a safe place as a backup option): Now, the MBP is running great (for its age), quiet fan except for high workloads and no runaway 99%-CPU processes.
Of cause, your mileage may vary, but I would suggest trying a fresh install from scratch and only install necessary programs. Don't rely on a TM backup.
By the way I had odd issues with CalanderAgent going nuts and using 100% cpu all the time after going to High Sierra. Dunno if anyones got a good fix for that? I managed to kill the calendar completely and am using google calendar instead now but it's not ideal.