Preview整个compersition,除非你有足够的RAM

When you preview a composition in After Effects, the more detail and precision you want to
see, the more slowly the composition displays in standard preview and the more RAM is
required for RAM preview. You can control the amount of detail shown in either the standard
or RAM preview by changing the resolution, magnification, and preview quality of your
composition.
You can also limit the number of layers previewed by turning off the video switch for certainlayers, or limit the number of frames previewed by adjusting the composition’s work area.

不至于吧，这么多的RAM才几秒。你的compersition尺寸几多大。
AE不用硬盘作缓存，应该是考虑到速度问题。combustion也这样。
这是combustion的RAM Requirements，AE也应该差不多。

The playback controls in combustion provide real-time playback of your clip without exporting the clip to a file. When you first play the clip, each frame is cached in RAM; the next time you play the clip, the frames that were cached play back in real time。
You can also use the Render to RAM feature to preview your work in real time.
RAM Requirements for Playback from RAM
As a rule of thumb, one frame of D1 NTSC video is about 1.4 MB. Therefore, on a system with 256 MB of RAM, you can cache roughly 30 frames when you are working with five layers in a result composite. You can store more frames in RAM if you use the Render to RAM function.
Note: The RAM cache always stores uncompressed RGBA images.
For audio, each minute of stereo 16-bit, 44.1 kHz audio requires about 10MB of RAM; a mono file would
require only half of that.
The following list describes typical image sizes and the cache RAM required for one frame:
• Large Multimedia (640x480) = 1.23 MB/frame
• D1 NTSC video (720x486) = 1.4 MB/frame
• DV NTSC video (720x480) = 1.38 MB/frame