Hey, I have exactly the same problem. I'm also from central europe and can play over night and in the morning, but in the noon/early afternoon the game starts to lag and is unplayable. Tried to play it on a second pc, but no difference. No matter if I try to play vanilla dow2, last stand or elite...
Have you played on 13/14.01 right after the update? I just played it on the 13th in the late evening for a short game to take a look at the "great" changes and even then it seemed to be a bit laggy, so maybe this is causing the problem. But so far I haven't found a solution
