The snow in the slopes is a simple particle which you can walk through and it does not interact with you in anyway.
Even if the snow on the ground did pile up and than went away after X amount of time, for that amount of time that it is there will still cause lag.
Heck, I start lagging with around 5 water particles in Gmod(Source), and you're asking for like 50 in Blockland(Torque)
Not going to happen. :P