Lately I've been noticing some odd mouse lag, where the game registers clicks where the crosshair was aimed a few frames ago. You can actually see the selection box lagging behind the crosshair by a few frames (this lag was introduced between 14w21b and 14w25b). I believe the responsiveness of the crosshair itself hasn't changed.

It's hard to account for all the different factors leading to an experience of a certain type of lag, so I devised a simple test: circle around mobs and try to hit them. If you keep the crosshair continuously centered and the game is consistent, this should always hit. In ≥14w25b you can do this for a while to no avail; you need to lead by a few frames to hit reliably. ≤14w21b is much better, perhaps off by one frame.

To reproduce most effectively, circle around tiny slimes at full speed creative flight and try to hit them. This is to reduce the temporal size of the target, making timing issues more apparent. The problem seems to be limited to mouse movements, which circling takes care of.

My videos were recorded at 60 fps in 14w33c. The fps limit was set to 120 fps. My machine actually hits 120 fps under these circumstances.

See:
Code analysis by jonathan2520
Secondary code analysis by caucow98 (prefer jonathan2520's, though)