that's because of the lighting engine - afaik

if you want it to compile fast change the following options in the compiler window :
- increase or max the "tesselate shaded"
- increase or max Rad. subidvision
.....

just move your mouse over the sliders wait a sec and read what shows up ->
like for example :

Light calculations -> "low = fast build, poor lights/high = slow build, nice lights"

but mind that altering these values means in most cases poor lighting

cheers