You may have noticed that since the 2014 release, render time has greatly increased for After Effects CC 2014 compositions in Premiere Pro CC 2014 sequences when the main After Effects application is open.
There is a workaround: close After Effects when rendering After Effects compositions in a Premiere Pro sequence.
The After Effects team has fixed this issue in the upcoming version of After Effects CC 2014.1. See the “a few choice bug fixes” section of this blog post for more details.
Some customers have reported a similar issue regarding slow rendering of After Effects CC 2014 compositions in a Premiere Pro CC 2014 sequence (Mac OS X 10.9.x) even if After Effects is closed.
The problem is related to running an OS X feature called “App Nap.”
See this document for more details.