When moving with WASD if you spam the buttons too much they will stop responding until the key is no longer being used. This makes movement really hard and annoying in the game, especially with third party inputs.

I use a controller by hooking Minecraft up to Steam and using their controller support option, and it is very noticeable with that option. I can also recreate this by randomly pressing my WASD keys on my keyboard, so I know that this is a game issue, and not steam.

How to replicate, easiest option would be to map a controller joystick to WASD 8-way using steam and moving around in a semi wild way, but for keyboard and mouse, just spam the keys until one or more stops responding.