Vis = Visibility
Ie: What the game renders... goes from back to front, (use r_draworder 1 or gl_wireframe 2 to see the reverse of the normal render)
Blocking vis can have advantages - less wpoly = more room for detail
more wpoly= lag
+799 wpoly = software can't manage, starts dropping polys = looks crap and lags
ALWAYS USE -full VIS!!!! for final compiles
... there are tons of stuff on this..