Preview playback is broken. (bug)
Preview playback is persistently choppy. Audio stutters as does frame rate during playback. This issue is SOLVED by hiding the timeline, which is not a practical solution every time I need to do a RAM preview.
I'm in CC2018 15.0.1 as suggested by Adobe support on the phone - it behaves exactly the same in 15.1.1.
CC2014 works absolutely fine in comparison.
I’ve reset VRAM and PRAM und also upgraded my OS. Nothing fixes it so far. I’ve tried enabling/disabling Mercury Transmit in prefs to no avail.
I'm on a Mac Pro Tower running High Sierra, 12 core 3.46ghz process and 48GB RAM. I was running El Cap with CC2017 but preview was still broken.
Can someone please contact me about this? Thanks. Support had no other solutions and suggested I post here.

The latest (November 2019) release of After Effects includes major improvements to our preview architecture. You should see reliable, real-time playback of cached frames.
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Brittnell Anderson commented
The latest version (17.0.0) is unfortunately even worse for me. The interface locks up, and the only way to stop playback is to force quit the program via Windows Task Manager. I've posted to the support community (including an example video) here: https://community.adobe.com/t5/After-Effects/After-Effects-2020-Preview-locks-up-program/td-p/10723016
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Arien De Beer commented
No feedback on this yet?
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Mikael Ovesson commented
I have the same problems here on PC win 7. Even without any video in the comp. Audio playback is consistently choppy and occasionally slows down to a crawl. I might add that I have a 16 core xeon E5-263. 32 GB ram, SSD cache disc etc. I've also been working with this professionally for 15 years, and audio works perfectly everywhere else on my machine. I also know others who have the same issue, so yeah. This is an AE bug. Please get this fixed asap.
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D commented
Could someone please confirm if a reply is possible? Thanks.