But if Ableton install is still the same as it was when it ran smoothly, I don’t think it’s got something to do with Ableton.
You should try ruling out things.
You talked about projects that ran smoothly, are now consuming all CPU. Has something changed in the VST’s that are used? Has something else changed, or is something else installed on your machine lately that might be causing this?
Can you get the same CPU state when only using Ableton and its internal plugins?
If you’re near an Apple store you could always take it to the genius bar and get them to have a look.
I have no idea what happened, but now my CPU is running at 20% in situations where its been at 80%. Not sure what happened, but its back to normal. @Daniaan, Phil, and everyone else who commented: Thx so much for taking the time to help w/ your suggestions 
In case any of you run into a similar situation, here’s what jake perrine had to say about it this morning: “Likely your computer had a background task it was doing. I know macs can rebuild their search directory and take hours to do so. Use activity monitor to tell you what your computer is thinking about.”