After Effects keeps a cache of rendered frames. That’s the green bar that you refer to.
When you make a change, After Effects recalculates (re-renders) the frames of the composition to accommodate that change.
If there’s part of your composition that you’re done working on, then maybe you should consider pre-rendering it, so that After Effects doesn’t need to keep re-rendering it every time you make a change somewhere else.
from “Pre-render a nested composition”:
“If you have a nested composition that you do not expect to work on further, you can save time during each rendering operation by pre-rendering the nested composition into a movie and replacing the composition with the rendered movie. You can still modify the original nested composition, because it remains in the Project panel. If you make a significant change to the original nested composition, just render it again.”
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Todd Kopriva, Adobe Systems Incorporated
putting the ‘T’ back in ‘RTFM’ : After Effects Help on the Web
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