I made a thread some days ago about this with a video showing exactly the problem I'm experiencing. After some playtime performance seems to deteriorate. FPS go from 144 to 10-30, when you aim sometimes FPS are getting back to normal but when you stop aiming it's back there waiting to ruin your experience. I can't find a pattern as to what is causing all of that, sometimes it happens on grineer ships, sometimes on corpus or in a free roam map. Restarting the game seems to fixing the issues.