-Join a heavily stressed server or make your own (a thousand AI fighiting each other should seal the deal)
-Your frames must be around 20 or 30 (under stress)

*1: Get in a car (for more obvious results);
*2: Get some speed so you can better notice the drop;

1. Press random key (any keyboard key will do) (causes frame drop);
2. Release random key (FPS will come back to stable);
3. Press 2 random keys (bigger drop than the 1st time;
4. Press 3 random keys (nearly frozen screen);

[Always press the keys at the same time, release at the same time]

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.