This happened to my teammate almost every game before he fixed it by deleting his coh2 folder (not the main one but the one in documents) and verifying game files. Not sure if that really fixed it but he was fine since then so it might have helped.
After I updated to the 64 bit about 1h after release, I had 15 FPS on all maps after about 1 minute of gameplay (RX 580, 16 GB RAM, Ryzen 5 1600). Low settings, same shit, highest possible settings, same shit. Then, out of the blue, the FPS just stabilized. I didn't verify, didn't reinstall. Literally tested around 20 maps one after the other on all possible settings with Vsync off or on (I later thought that turning Vsync ON was what fixed it but it wasn't).
So, I shut the computer off, and went to sleep. Woke up, studied, and after an hour or two, went on to test again. Opened COH2. Same thing. Of,on,high,low, each and every map I tested I had 15 FPS after about one minute.
Again, I turned off COH2 and went to cook myself some lunch. After an hour and half, I wanted to give it one last try before reinstalling. Lo and behold. The FPS went to 80-90 and was as smooth as butter. Literally the computer was idle for an hour and a half and somehow the FPS got fixed.