I was in battles with the spider queen and with Letho also and I noticed that some keys lag. I have 40+ FPS, certainly that does not the answer.

When Geralt got a hit and immediately after I want to press 'Q', the Witcher is simply standing and doing nothing. At the spider queen I was able to notice this, because the queen didn't attack me for 4-5 seconds (maybe an other bug) and until that time I could repeatedly press Q-Q-Q-Q-Q-Q. And nothing. After I moved a bit, it worked again.

Now, when I'm in battle with Letho, I tried to select an other sign, but didn't work. I clicked on the sign and nothing changed.

Please, fix it, because this bug is certainly not in connection with the gaming performance (every setting is on 'high', with 30-40 FPS).

Note: You should have posted this in the questions/problems category and/or another lag related discussion.

Let's narrow down your problem.
So you say, that after getting hit, none of your signs work.
Does this happen every time?
What happens if you use another save?
Do only signs get disabled?
What kind of hardware and driver do you use?
What happens if you replace your keyboard and mouse with another ones?
Does the problem occur only on your computer, or your buddy's too?

Anyway, you always can write to the support team. I forgot their email though.

I don't know if this will be of any help but I also get unresponsive keys at times that can usually be fixed by pressing escape to go into the menu and then going back in game.

Just a theory but I think the bug is related to vigor depletion. When you're out of vigor you shouldn't be able to parry or use signs, instead sometimes it paralyses Geralt so he's stuck in the spot. The only key that responds in such situations is Space, make him roll and he'll come to his senses(double tapping WASD won't work). Like I said just a theory, I'm not anal retentive enough (ok, I'm lazy lol) to experiment.

Call me crazy, but I think the bug is bad programming.