I think I had the same problem as you (similar hw and setup). Do you have a discrete GPU? If so check the link below out. For me, this behavior was due to OSX flip-flopping between discrete and internal GPU. Once I set it to discrete all the time (I'm always plugged in) the problem went away.

https://support.apple.com/en-us/HT202043

Seems to me a different issue than what people are complaining about atm... but what do i know, might be one and the same.

Edit: very weird... Looks like this setting was reverted for me. I just updated from 11.? to 12.0.1, so I wonder if the installer undid some of my changes. No performance issues, but I've literally just my computer on for the first time after the upgrade.