Tested 40000 zones in one scene. The client drops from >1000 fps to ~160-300. Server is giving lag spikes, likely due to processing going on in the background.
Deactivating the zones made no difference.
When I drop ghosting, I still get the lag spikes, but fps stays around 850-900.
Testing @20000 & 10000 show shorter spikes, but they still appear at the same rate. At 5000 It's almost impossible to detect.
Its easy to go over 5000 bricks, so... per brick rain-block just isn't practical.
Using a tool to apply it should be restricted, so some random mumbo can't apply it to a cube of 1x1f and lag the server.
I believe the lag has little to do with rendering. Adding extra zones affects frame rate, but it has no where near the same effect as static shapes.
I tried the same tests with Triggers instead of PhysicalZones. It takes about 2 times as many Triggers to get the same amount of lag. And both are similar in that they must keep track of what objects are inside of them.