Thank you for the quick response.
I had restarted my PC, then the game refused to start up at all, another restart and it runs fine now, at 40 fps again, so god knows where the problem was.
I do not understand though why you gave up on backward compatibility, is it so hard to write some convertor for the saves? I mean even Microsoft's Word can open older documents ;o) Imagine that I start playing the campaign again and after some hours into the game new patch will come out and all my progress (all the time I invested) will be lost, again - it is something I am not looking forward to.