I'm having issues with both RAM previews and rendering. iMac Pro (intel) with 32GB RAM.
1) RAM previews nearly always cover a smaller portion of my work area before they stop. In larger and more complex files they sometimes don't work at all. I always clear the cache, so it's not that (but that helps a little). Previous versions of AE on the same computer gave me longer RAM previews.
2) When rendering my latest project, it slows to almost a halt midway through the rendering. When I open Clean My Mac and select "Free Up Memory" things speed up greatly, but this mean I cannot leave long renders unattended--this is a drag! When I look at the memory usage it's only about 13MB so I'm not maxing out the RAM (as far as I know).
Make sure you have enough ram allocated in Preferences. Multiples of 4 GB are best. If you have 32, try 28 or 24 for Adobe Apps by adjusting the amount reserved for non-Adobe apps.
Check your cache settings. If you have less than 500GB available, a good starting point would be to allocate 25% of the available ram on the drive. 33% would be the maximum.
If that does not help, open up the Creative Cloud app, go to the All Apps page, click on the three little dots just to the right of the Open button for After Effects, and select "Other Versions." When the options pop up, select the previous build and re-install. If your previews return to normal, you can try updating AE again and see if the fix holds.
Let us know if any of those suggestions improve your preview performance.