(MacOS) Rendering slows/pauses when AE is not forefront or screens go to sleep

Is anyone else experiencing this? Sometime after my screen saver runs for a bit (it's not immediately tied to the screensaver), my screens go to sleep, or even just switching to a different application, After Effects 2019 slows rendering to a crawl. Just starting this thread in Chrome (5 minutes) created a three minute pause where AE didn't render anything. As soon as I switch back to AE it'll render a few frames, and keep rendering...till a few minutes after I switch back to any other application, where it slows/stops again.

Last night I set a number of sequences to render, all of them 2 seconds long. I set the screen saver to turn on after 1 hour, and display sleep after 3 hours, and made sure no other apps were open. Based on the frames rendered, an hour and a half after I left my computer (so not necessarily directly tied to the screensaver) After Effects dropped to about 3 frames every 4 hours or so. It took between 1 and 19 minutes to render each sequence...except after the monitors locked, it spent 14 hours barely rendering at all. Once I unlocked the displays, it finished that one sequence in a matter of minutes. It's now slowing down or pausing rendering every time I switch to a different program (I currently have four apps open - After Effects 2019, Chrome with 6 tabs, Activity Monitor, and Versions for SVN).

When at the forefront, After Effects is only using 1 or 2 out of the available 12 cores, and about 56GB out of the available 64GB RAM (according to the activity monitor it has about 11GB in compressed memory). CPU usage drops to about 1-2% of a single core when AE is not forefront. I assume this is what's happening when the screens lock as well (since it technically doesn't technically entirely stop rendering...it's just incomprehensibly slow).

Hardware: 2013 Mac Pro — 2.7 GHz 12-Core Intel Xeon E5, 64 GB 1866 MHz DDR3, dual AMD FirePro D700 6144MB, 1TB SSD

Software: MacOS High Sierra, version 10.13.6 (latest update for this version, I can't upgrade to Mojave yet due to other software requirements)

After Effects: CC 2019, 13.0.3 (latest update)

Any ideas? I've never had issues with MacOS throttling background apps before (I just rendered a bunch of 3D assets the other day in a different app without a hiccup), nor have I had issues before with AE killing itself when it's not the centre of attention...which appears to be what's happening now?