Sometimes the Rad step looks like it has stopped responding, but it is actually just taking a long time to do its thing. If your PC is low on memory it can take an unbelievably long time. When the compiler runs out of available RAM, it starts swapping pages to the hard drive which is extremely slow. On my old low-end PC, I've seen some compiles take 8 hours. During that time, it only 'appears' that the compiler has stopped responding. Ministeve is right, using a batch compiler will free up the memory that Hammer uses, you just have to remember to shut Hammer down before using the batch compiler. As a side note, you don't need to run rad every time you compile, you can turn it off if you aren't adjusting the lighting of your map.