When standing still on slime blocks the screen starts to flickering and one value in "F3"-screen starts to change between true/false very fast. I think the side effect of this is the collapse of the FPS. I forced a crash and added the crash-log.

Ways to reproduce:

1) Place four or more slime blocks one the ground
2) Step on them and stand still
3) Press "F3" and check the true/false value and the FPS value