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.
see: https://forums.creativecow.net/readpost/2/992912