@lukeofguitar, I used to use Logic Pro X still do from time to time. When I was working on tracks individually I disabled the ones I was not working on. This does help, it turns off the plugins in the disabled track(s), but it is a pain when you want to hear the tracks in perspective. I always close all other apps on my iMac. I also turn off WiFi and if you have DropBox shut that down too as it's a mini memory hog!
The other thing I did was to have an empty instrument track at the bottom (Named: CPU SAVE) that sends only to Stereo output. When you go to play back, select the track, and then begin playback, it seems to level out the cores, might work for you, might not, give it a try. If you need more explanation, let me know and I'll post pics.
Hope it helps,