There are a few different ways to do rain, some more efficient than others. You could have individual droplets that spawn randomly, - which is extremely performance hungry - or animated sheets of rain - which are a pain in the ass to place. There's also the option of having very large groups of raindrops that fall, so there are less objects, but then there's the problem of having them go right through terrain.
Edit: and don't even try to use physics man, that's a nightmare.