Hi,

does anyone know how to speed up the Building Process because for testing the new part in my level, I always wait 10-20 minutes. But the level isn't very big and I found that the architecture isn't very complicated.

Check out the Levels Forum.
You'll find many many posts on how to decrease your portals and compile times.
Short of modifing your models and terrain, the best way to decrease compile time is better hardware.

Check out this post Here