I was having an average perfomance before the patch, nothing too bad, but far from perfect. But now, after installing the Beta, the game is almost unplayable. I have FPS bellow 20 all the time, with drops to even lower numbers on certain parts of the map.

My spec: i5 4460 3.2GHz. 8GB, Geforce 750M (2GB).

I know my GPU is not that good, but my point is that before the patch the game was running better.

Patch might have reset your graphics options go over your options in the launcher again.

Yes, it was my first thought...but even playing on low (I used to play on high before the Beta) the perfomance is really bad.

Edit: definitely there is something wrong with the beta, I opt out and went back to the previous version and the game is running fine again.