Subversion is comparing the "pristine" version of each file with the file that is currently in the working copy to figure out which files where actually modified and needs to be committed. (The pristine version is how the file looked when checked out from the server).
I don't know the exact details but I believe this is only done when files have changed date/time compared to the checkout. What happens if you do an SVN Update and then immediately SVN Commit - does it hang the same way?
How many files do you get in the commit dialog when the dialog starts to respond again?
There is probably not very much you can do about this, when you do a blank commit like this it has to figure out what has changed. However if you have a lot of large binary files in your repository there was a recent interesting discussion at the
d...@subversion.apache.org mailing list about doing away with the pristine files, which would do away with about half of the processing you experience.
Kind regards,
Daniel Sahlberg