Simply the remove grain does a ton of calculations to clean up the footage and it’s doing it on every pixel. And it takes a long time on most computers.
Couple of things you can do. One is investigate network rendering. You install the AE render engine to every computer you can, then setup a network render. Definetely going past the AE Basics stuff, have to open the help to do it.
Second, you said “if there are changes”, assuming you are compositing, you should probably precomp all the footage with just the remove grain effect, then render out and with the choice “replace footage”. If there are changes you won’t be re-rendering the remove grain effect.
Johnny Cuevas, Editor
http://www.ckandco.net