When a block is updated it can cause a massive drop in FPS.
This affects redstone circuits that use a lot of blocks the most, but even one piston moving can cause the framerate to dip drastically, even on a powerful machine.
I can consistently recreate this problem by opening a new world and creating a small redstone circuit.. (A 2x2 piston door should be enough), and while holding down ALT, open the F3 screen. Activate the circuit and watch the framerate spikes.
This can also be seen when chunks are updating in general, which may or may not be the cause of the massive framerate drop when changing dimensions.
(i7 860 @ 2.8 Ghz, 8 GB RAM, 2GB VRAM GeForce GTX 760.)
Screenshot at 16.10.19 is of me just opening and closing a door rapidly.
(Sorry if this is an issue already reported, I didn't see any that related when I searched)