Upon using a sword on blocks while in Creative Mode, the client starts experiencing movement lag for a few frames for every swing; it affects walking, sprinting, sneaking, flying, swimming(not shown in video, but it still occurs) and, if timed correctly, a swing can reduce the jump height of the player.

Tested on 1.14.4 Singleplayer, with and without resourcepacks and/or shaderpacks. All swords are affected. Unsure about Multiplayer.

Video Link (too large to attach)

(Side note: Sneak animation is not rendered while flying)