Done with the inclusion/exclusion filters
The tree selector widget is a piece of art even though it doesn't look like one :) It is basically a front of an asynchronous parallel disk scanner that dynamically rearranges its scanning queue to first scan subdirectories that a user might be accessing next.
In other words, if you expand a directory, then the scanner will postpone scanning anything else and focus instead on all its subdirectories. So if you are making your way through a massively populated disk, it will try to pre-load what you are looking at and scan the rest on a lower priority.
Also, since the widget starts showing tree contents right away, it allows quickly navigating to and excluding a desired item
needing to wait for the entire scan to complete. Again, quite a bit of speed up with huge trees.