To reproduce, run a BWS like this one and go to the Root section. Run your mouse from top to bottom over the frequency graphs, there is a lag as the UI tries to calculate all the mouseover info on each graph, during which time the interface is unresponsive.
Running L5.0a Beta1 on Mac 10.8.2
I am able to reproduce this issue, and will report it to Development.
This will be fixed in 5.0b RC 1.