FPS fluctuation

Created a superflat world with a single layer of glass. I went into the nether, and I noticed that my FPS started to fluctuate greatly. I'm running it at almost, if not minimal settings, tiny render distance, fast graphics, minimal particles and no Vsync. I'll generally get about 1000-1200 FPS, but the second I start moving, jumping, or anything, it will drop to 0 for about a second and fly back up into the hundreds.

(I'm about to update java, once I finish this, will update with results.)

What happened?
-Severe FPS fluctuation in Nether

What I expected to happen?
-No fluctuation.

Steps:
1: Superflat World, Customization code "2;20;1;village"
2: Seed # 8477772699963483377
3: Make a Nether Portal directly in front of you
4: Walk through it.
5: Check for FPS fluctuation