I'm rendering a complex many-layered sequence. When I restart the computer and start rendering afresh, the render time per frame averages 30 seconds. Then, after some time (45 minutes, say), the render time per frame shoots up to 4 minutes. The 4-minute frame differs in no way from the 30-second frame. If I restart the computer, the render time returns temporarily to 30 seconds per frame. Can anyone tell me what's happening exactly?
I'm running AE6 on a Mac 2x2.8 GHz Quad-Core Intel Xeon with 16 gigs of RAM.
Without knowing any of the specifics of the Composition (frame dimensions, effects used, type of footage), nor whether you're using multi-processing, I suspect that you've filled the RAM buffer.
Launch the Activity Monitor and check the RAM usage as the render progresses, my guess is that you're running out of RAM and the Mac is using VM which is several orders of magnitude slower than RAM, if so you'll see a large number of Page outs.
16GB isn't very much RAM these days for After Effects, one option is to use the Secret Preferences to have AE purge memory after X number of frames have rendered.