You can adjust priority on the compile tasks (vvis/vrad) through right-clicking on them in the task manager.
I strongly advise compiling maps outside of Hammer; it's a memory hog and seems to cause system instability for me the longer I leave it open. You can do this with a Windows batch file and it takes about 2 seconds to write. Here is the code (external link because it doesn't fit in the forum's formatting)
http://pastebin.com/ZwRgvwQvAll you have to do is fill in the paths specific to your needs, cutpate everything into notepad, and save as a file with a .bat extension. You also have to compile the map from within Hammer to start, but only with VBSP, to generate the .bsp file itself; which should take no longer than a minute or two.
The advantages of this method are that you will experience faster compile times and you should get a real-time update of how your compile is progressing. The only disadvantages I have found so far are that...you have to deal with Windows batch files