As you work on a composition, After Effects temporarily stores some rendered frames and source images in RAM, so that previewing and editing can occur more quickly. After Effects does not cache frames that require little time to render. Frames remain uncompressed in the cache. You can control how After Effects stores images by setting image-caching preferences.

After Effects also caches at the footage and layer level for faster previews; layers that have been modified are rendered during the preview, and unmodified layers are displayed from the cache.

Blue bars in the Timeline panel mark frames that are cached to disk. Green bars mark frames that are cached to RAM.

Choose Show Cache Indicators from the Timeline panel menu to turn the cache indicators on and off. Showing the cache indicactors decreases performance slightly.

When the cache is full, any new frame added to the cache replaces a frame cached earlier. When After Effects renders frames for RAM previews, it stops adding frames to the cache when the cache is full and begins playing only the frames that could fit in the cache. The disk cache is not used for RAM previews. If disk caching is enabled, After Effects can store rendered items to your hard disk when the RAM cache is full during standard previews.

The RAM cache and disk cache are automatically purged when you quit After Effects.

To purge the RAM cache and disk cache, choose Edit > Purge > Image Caches.