EDIT: After testing a little bit more, the more keys you pressed the worse frame lag is!
No need to be in a car, infantry works as well... If you are moving with keys pressed you get low FPS, when you released them it's all good.
Mouse buttons appear to not affect!
The rain has cleared, the fps drop is still in place, will reopen Arma to see if it is fixed.
Simply ending and reopening the scenario appears to have fixed it.
Unsure whether the rain caused it or not.
Issue happenned again on a my unit's MP server, started affecting me after 1 hour of mission.