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This is happening to me using all of the builds for NB6 from the last couple weeks or so (I upgrade often). I see this when trying to do something involving the file dialog...like when you add a file to a library for example. One core will show 100% usage and NB will not be responsive (other than paint refreshes I think). After 30-60 seconds, it comes back, but as soon as you use the dialog again (to drill deeper into the directory for example) it starts over again. It took me four or five minutes just to add one file to a library I had created. Other than that, the performance seems to be pretty good as of late.
This is a duplicate of bug #109703 ("Netbeans 6.0 M10 FileChooser is too slow"). *** This issue has been marked as a duplicate of 109703 ***