Hey, the first thing i noticed after updating to 2015 was that the RAMPreview did not stop by clicking somewhere in AE (which it did before). In order to recreate your problem, i quickly generated a testscene with a lot of DOF, which usually takes a lot of time to render. As before “2015” the first run was slow, not realtime in order to render the frames into RAM. But as long as i did not change anything, it ran in realtime during the second runthrough. When i change something during RAMPreview (which was not possible before), AE needed to calculate again, resulting in a slow playback again.
Only sometimes i noticed the RamPreview not running in realtime during the second (or third) runthrough, although i did not change anything…
cheers,
kws