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do you have overlays like RTSS or other DRMs like UPlay/Origin overlays when playing the game?

if yes, then multiple overlays can cause such behaviors, best thing is just to keep one overlay active at once.

other possible cause is AV software might cause issues with overlays, so try to add steam to exceptions list.

hope this helps.

Had that yesterday where it was really slow, i rebooted or restarted and it was fine again.

Nope, on my configuration works very well :-)

Mine has not been working recently either.

I have that problem every goddamn day. Sometimes it works like a charm, sometimes it doesn't. Sometimes I need to restart the game, because it doesn't work at all.
Only solution is to hate Valve and deal with it I guess.

yep, sometimes i have also this problem