I believe the fps drop can be there for many reasons. Even in more popular games there's always players who deal with them. In BNS however there is a huge part of the players who is dealing with poor performance in systems that are considered an "overkill" for the game. If you see the recommended requirements to run the game it's pretty ridiculous, almost anyone should be able to run it decently.
As for me, I have tried many things to solve the problem and nothing worked well. My next attempt is going to be the installation of the game on Windows 7. Currently im running 8.1 pro and