Before anybody can give you any kind of useful answer, you need to provide more information:
• version of AE
• operating system
• amount of memory
• amount of storage
• type of footage: its wrapper (mp4, QT, AVI, etc.) and codec (H.264, ProRes 422, DVCProHD, etc)
• exporting or using the AE Render Queue?
The more information you give, the better information you get.
I'm using After Effects CS5.
The footage I'm using is in .mov format, although to be honest I'm not sure about the codec (I imported it straight to After Effects from my Canon 7D's memory card? I think it's H.264)
I also have some PNG sequences.
I used the AE Render Queue.
The computer I'm using runs OSX.
Processor Name: Quad-Core Intel Xeon Processor Speed: 2.8 GHz Number Of Processors: 2 Total Number Of Cores: 8 L2 Cache (per processor): 12 MB Memory: 4 GB Bus Speed:
First thing to do is transcode the H.264 to DV and then build your DV comp in AE. You're spending hours doing the transcode on the fly inside AE and that's terribly inefficient.
If you must maintain the size of the Canon footage, then use ProRes422LT.
You also should know that 4GB is sufficient to run AE, but that's about it. AE is a memory hog. If it were my machine, I'd say 8 gigs would the minimum, and I'd only consider using multiprocessing once I reached the 16GB threshold.
Then there's Open GL. If you use it to accelerate rendering, turn it off. Leave it off. It just doesn't work as billed. With the exception of a tiny handful of pricey third-party plugins, it doesn't do any good, and mostly does harm. Adobe employees don't use it, which also should be a big red flag.