Original Steam forums post: here.

The following are copy-pasted details of the issue from the Steam discussion linked above.
Quote Originally Posted by Mister Musashi
"Getting a bit annoyed when half the time I have to mash W for 2-3 seconds before I'll start moving, though holding both left and right click seems to always work. Also sucks standing around mashing M to open the map though I suppose there's a button I could click. At some points after a bit of time everything will start to respond as they should, but not for long. The Steam Overlay also takes a long time/numerous attempts to open and close.

I can only assume that it could be because stuff is loading...? I don't know... I've turned down some graphical settings despite NVIDIA GeForce Experience's protests but that didn't seem to really help, either. Is this a known issue? Is there a fix? And should I just try playing with a gamepad, assuming it avoids this issue? (Xbox 360 controller is all I have available so I don't know if that would work... would it?)"
Quote Originally Posted by Mister Musashi
"[...]everything else responds just fine everywhere else, it's just been the keyboard specifically with this game. The game doesn't lag and there's no input delay for the mouse and there's no visual lag. Also, I'm noticing that if I'm holding left or right click and then pressing the numbers of the hotbar it works every time (or at least nearly)."
Originally posted by Asphe:
You're also probably only noticing this 'problem' in game where you're constantly pressing certain keys.
Quote Originally Posted by Mister Musashi
"[...]I've never had this problem in ANY other game thus far - TF2, Vindictus, Guild Wars 2 (beta), Elder Scrolls Online (beta), Planetside 2, Warframe, the Borderlands series, Titanfall, etc."
Originally posted by Freyar:
It also depends on how many other keys you are holding too.
Quote Originally Posted by Mister Musashi
"It does the problem when nothing else is being pressed, though. If I hold down one of the mouse buttons it appears to work correctly for the most part... it's strange. It's about the only way I can use the number keys to use my attacks on the basic hotbar. And if I click both keys to start running, press and hold W, then let go of the mouse keys I can run and turn using WSAD like almost normal."
Originally posted by Freyar:
Do you have overlays running on the game? Steam, etc?
Quote Originally Posted by Mister Musashi
"Yeah, I have the Steam overlay running. It's never caused the problem before, but the overlay itself has the same problem as FFXIV, so... is that it, ya think? I can try disabling it.

EDIT: Didn't solve a thing :c"