This is one of the many reasons After Effects loves RAM! It depends on the length and complexity of your comps and how much RAM you have. For example, you might create RAM previews of Compositions 1, 2, 3, and 4, but when you reach Composition 5, it may automatically purge the RAM preview from Composition 1.
Eventually, when AE fills the RAM cache, the oldest cached frames will be purged automatically. If you are working in many compositions at a time within a project, AE will continue to create RAM previews as you play each composition. RAM previews will eat up as much RAM as you allocate to After Effects (you can control this in Preferences, under Memory). That is, unless, you make a change to any parameter on any layer in your comp and then the RAM preview disappears and the process begins again, since even the tiniest changes causes After Effects to have to recalculate the altered frames all over again. Once After Effects has created the RAM preview, you'll usually get real-time performance on a second play-through. If your composition is complex, playback will be slow, down since the playhead cannot surpass the RAM preview. If the composition is simple, the RAM preview creation (the green line) will outpace the playhead, and you'll get real-time (or very close to real-time) playback.
When you hit play for the first time in a comp, think of it as a race between the RAM preview and the playhead (a race where the playhead can never truly win). It depends on the complexity of your project and horsepower of your system. Sometimes the green line grows quickly, and sometimes it grows slowly. to create temporary frames and stash them in your computer's RAM so it doesn't have to perform the calculations the next time you play the sequence. After Effects is calculating the math of all your layers, effects, masks, keyframes, etc. RAM previews are part of something Adobe calls the Global RAM Cache, which is (mostly) automatically managed. When you first play your timeline in After Effects, you will usually see a green line begin to grow over your layers. Why is this? The growing green line (i.e. But usually, on the second play through, it plays smoothly. Common to most motion graphic applications, After Effects will not skip frames - it will play every frame of video, even if it has to slow down playback to do so. If you have a 10-second composition in After Effects and hit Play, it may take 10 seconds, or it might take 17 seconds, or it might take 2 minutes and 43 seconds. This is not always the case in After Effects. In Premiere, if you have a 60-second sequence and hit Play, the playhead will reach the end of the sequence in exactly 60 seconds, even if Premiere has to skip frames to do so. To get started, one thing to understand is how playback differs between Premiere and After Effects.
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But much like the Media Cache in Premiere, understanding a little behind the scenes action in AE can fix glitches, conserve system resources, and free up valuable hard drive space. Underneath the hood is a whole other story. After Effects is a challenging enough application to learn from a forward-facing perspective.