Many of us have experienced strange drops in fps and general performance over time, whether whilst playing or over time between sessions.
Many things have been theorised. None have been confirmed. Basically the game code is terrible and there's nothing we can do about it.
It's another case of 'wait for a patch'. As for me, I give up - I'm sick and tired of the way the game (barely) runs. I'll come back in 6 months and see if anything's changed.
