Just wanted to add my experience to the list of people with a now unusable ram preview. Extremely frustrating (and I own a studio with multiple copies all having similar issues). Furthermore, it seems the "cache before playback" functionality is missing from the interface? Can't playback realtime audio / video (audio and video stutter). 4ghz imac, 32 gb ram, el cap (and AE has been updated). What the hell is going on? How does each release seem to somehow get worse? we'll have to use 2014 for now as it's not in a production ready state currently.
if it's not obviously, AE CC 2015
Lots of fun when the software developer has you by the short hairs, isn't it?