I've been trying to play this game but my mouse moves so slowly it's making the game nearly unplayable because I just can't turn around.

Is there a keyboard command for turning left or right rather than just moving sideways?

Q an E turn around!

Aha, that's super. Thanks!

Can't believe I didn't try that myself... :)

You're welcome. It's a stupid bug, that has never been patched and only appeared after the last update of the game.
Using the old version isn't a solution either, as it includes a major memory leak bug which regularly crashes the game.

I had the same problem, but wanted to run full screen. I was able to fix the lag by forcing vertical sync in the Nvidia control panel.