The entire preview approach changed in Ae CC 2015, as part of Adobe’s modernization of Ae, to try to improve performance.
Two major changes:
1) The UI and the renderer have been split into separate threads. This really improves responsiveness; the renderer no longer blocks the UI (and vice versa).
2) Preview can happen without rendering the preview range to RAM first. When the comp is simple (or highly GPU-accelerated), you can preview in real-time, right away.
If you want to emulate Ae’s classic RAM preview approach, check the “Cache Before Playback” option. (I think this is enabled by default on Numpad-0.)
Walter Soyka
Designer & Mad Scientist at Keen Live [link]
Motion Graphics, Widescreen Events, Presentation Design, and Consulting
@keenlive | RenderBreak [blog] | Profile [LinkedIn]