Lag seems to happen only in VS+PS 3.0 and in 2.0 it seems to work fine... This means that I won't be able to use spotlights -__- Or maybe I'll figure out how to get rid of this lag in 3.0 somehow. OR I'll need to switch to per pixel lighting...

Edit: nahh... it actually seems, that it's related to the way I add calculated lights in VS to the texture in PS. If light calculations are complex, it lags out. So the one with spotlight won't work for me, if I go with the current setup. If I use default.fx's DoLight function for light calculations (without spotlight) it works pretty normally. I guess I got to think of a better way to pass lights from VS to PS.