[ Edit: 31 March 2015: I have just upgraded to Mathematica 10.1.0.0 and the problem (bug) is no longer present.]
Mathematica 10.0.1.0 Mac OS X 10.10.2 x86 MacBook Pro late 2013
Consider the following simple Animate enclosed within a Subsubheading cell
Animate[Plot[Sin[a x],{x,0,2 Pi}], {a,-1,1}]
After evaluating the input cell, the animation runs very smoothly and can be stopped and started without altering the performance.
However, if the animation is stopped and the heading cell is closed and then re-opened, then upon restarting the animation it is very jerky. The same happens if the animation is left running when the heading cell is closed.
On the other hand, if I stop the jerky animation and control it manually using the slider it is beautifully smooth!
This thread discussed a similar issue Animation is jerky, but slider is not
but the cause was found to be with Windows 7: "Further investigation showed this to be a problem when running Mathematica as a user under Windows 7. Under Windows XP and other environments the animation runs smoothly." So this does not address the problem I have identified.
Any help would be much appreciated.