I have the same issue. After about 1h gameplay i had a massive drop of fps (before > 40, after < 12). The only thing which changes was, that we were 4 people instead of 2. It happend at NW Airfield.

About 12-20 FPS in cities, had no problems before 0.47...

i7 4770k 3,50 GHz
Geforce GTX 780
16GB RAM
Win 8.1 64 bit