Are you sure that's exclusively your problem? From what I understand and read there is minimal network traffic associated with playing solo and shouldn't impact your game play. You could be experience performance issues on your local machine and server side lag when you try to play co-op.

For my friends and Cabal mates, the Co-op is 100% broken and unplayable... however, other than random freezes and minor other issues, the single-player experience has been mostly a positive one (other than not being able to play if the servers are down or the internet connection is lost) . Although, now that I think about it, the issues in single-player are probably due to the game being weirdly forced to be online all the time. 

I have it in single player as well and it's 100% internet lag and not FPS. Plus there is no issues with my internet