Hi Gafgar, have you set the Surfaces of the Block which one the Player doesnt see to "none"? This can increase the Build time much !
If made a Level with ~1500 Blocks, some time ago. This map had ~25000 Portals and the Build Time was 10 Minutes (VC + LC = High).
And my PC is very old

AMD Sempron 2500+ 512 DDR.
cYa Sebastian