@justinbarrett said
That is a lot slower than in blender(not by a small margin)....
It's actually not, though. His method is probably not the quickest. I know little of Hammer 2, but I think he could've just extruded the door cutout inward. He didn't have to cut it on both sides.
But regardless of that, you have to keep in mind that there's more to it than just cutting that door hole. There's also texturing to do, which will be a whole lot quicker than in Blender. And then there's probably adjustments to be done, which will be a hassle in Blender, but not at all in the level editor.
You don't build levels in Blender, you only make 3D models. Even if you can do something quicker in Blender, your whole workflow is gonna be much slower and much less flexible.
I think this guy lays everything about it quite nicely: