A new option has been added to the Gameplay Options: Physics Precision Reduction. This is a number from 1 to 32 that affects how precise the physics engine is.
1 is baseline, higher numbers are less precise.
What does this mean? If you have low FPS and your character slows down while running for no reason, it may help to reduce precision by a factor of 2, 4, or even higher. Fewer collision checks will be made and your framerate may improve. However, there may be other unintended side-effects. Use at your own risk.