I use AE sporadically for various things, but I am by no means a skilled user.
I'm working on a project in PP and I built an intro in AE (26 seconds long - 23.976 fps). When viewed in AE (rendered within the program) - timing of animations is as designed. When exported to PP or exported directly from AE the project is still 26 secs long, but the animations slow-down dramatically (i.e. almost like slow-motion footage).
Timelines and export settings are all 23.976 fps.
I've noticed that this has happen in some of my projects before - what's going on? I just know it's something easy that I'm overlooking.
We can't know. You have not provided any useful technical information regarding your export settings or how you review your footage, including system information. For all we know you could be rendering your file to an uncompressed format, store it on slow drives or even have your playback speed set wrong in whatever tool you use. You need to be much more specific.