Hitting Blocks with a sword while in creative mode causes the entire game to stutter severely and the player movement is impeded.

To recreate:

1. Do /gamemode creative
2. Spawn a sword
3. Walk forward and strike the ground at the same time

Observe that the player speed and movement spikes severely.

This only seems to affect creative mode at the current time. This appears to also slow the player down but does not break sprint. It only happens while striking blocks with the sword while moving.