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If you are having troubles with your RAM previews being slow then your cache, not cash, maybe set to low. Here is a rundown of how to up your cache:

To set Cache preferences:
1. Choose Edit > Preferences > Cache (Windows) or After Effects > Preferences > Cache (Mac OS.)
2. Set any of the options below:
3. Enter an amount for Image Cache Size as a percent of the amount of installed RAM. (The default value is 60%; Adobe does not recommend values over 90%.) The amount you enter determines the maximum amount of memory After Effects uses for cached frames. The number of frames that RAM Preview displays is also limited by this setting: Increase it to get longer RAM Previews; decrease it if RAM Previews are jerky or halting because of virtual memory paging activity.
4. Enter an amount for Maximum Memory Usage as a percent of the amount of installed RAM. (The default value is 120%; Adobe does not recommend values over 200%.) This setting limits the amount of memory After Effects can use. Increase the amount if insufficient memory errors occur. You can specify amounts greater than 100% because virtual memory is not limited to the amount of installed RAM.

To purge the cache:
Make sure that the Timeline window is selected, and choose Edit > Purge > Image Caches.