linux - Ubuntu GUI slows to a crawl when doing CPU heavy stuff

For a clients site we have an Ant-build that is highly parallelized. The build consists of multiple GWT modules that are all quite heavy and built in parallel. The build takes about 5 minutes or so.

This question is not about the build itself, but about the difference in scheduling/graphics stack between Gnu/Linux+Ubuntu+Gnome and Mac OS X. The build is basically just a CPU-heavy JVM taking as many threads as it can. The difference I have noticed in this particular case is that in Ubuntu I can hardly move the mouse after triggering the build. The build takes up all CPU and this seems to cripple the GUI totally. I can only grab a cup of coffee. In OSX, this is not the case - the GUI is unaffected.

Why is this so? How does OS X's scheduling work to make this "better"? Is there something outside of the build itself I can do to reserve some power for the GUI? I'm looking for something more permanent than running the build with nice.