Hi there!
I was struggling with exactly the same issue, and it drove me nuts! To make matters worse, I have a job I’m working on, and NEED to animate in AE. So I searched the net. Disappointingly there was nothing on the net. So I searched and searched until I at last found the solution. And what a no-brainer it was. Guess sometimes we think we’re so experienced, we fail to check the basics.
Check your Ram-preview, on mine the FPS was set to 30. Apparently this happens if you mistakenly use the “ins” key on the keyboard:D
Now all’s fine, hope it helps!
Cheers!