MoP Client not being so key-responsive?

I decided to start playing MoP, but I found a really annoying glitch/bug, most likely client side.

Whenever I try to strafe or jump while I am walking straight with button W, it has 50% chance of just "not working" at all, meaning, it wouldn't strafe/jump for some reason, so I have to press it really hard repetitively to make it work.

Same thing happens with the escape button. It sometimes doesn't work so I have to press it really hard.

It definitely is not my keyboard issue as I have no problem like this on WoTLK client.

There definitely seems to be something more laggy/sluggish about MoP compared to WotLK, but I can't quite put my finger on whether it's the client itself or the server.

Reinstalled the client, problem persists.