Yes. Happens to me, in most saves. I thought it was a auto-speeddown in order to let other programs to work well :)
Just drop some stones to quicken the FPS. Also, when I do tpt.setfpscap when FPS is drop, it just speed up TPT a little
Also, drop some stones, and you need to wait until the stones disappear to quicken FPS
My thoughts: It don't work with my processor (which generated particles and disappeared (this is BRAY) very often)
That means, this problem is because particles does not disappear for a long time
And so the memory used by the particle array does not free so it is probably because the unused space in the particle array is only free when a particle disappears
Are you sure you left it there for more than 1 hour?
No, it says 10 minutes.