there are 2 problems:
a) ntfs: if you have enabled support for 8.3 file names, then windows has to come up with unique 8.3 names. it "attaches" these names as extra names and if the file names in a directory are very similar, than the algorithm to come up with unique 8.3 names has a lot of work to do. the way the algorithm works is explained very well in "windows internals 5th edition, page 946-947".you can disable that feature, checkout http://support.microsoft.com/?scid=kb;en-us;121007&x=15&y=12.
and keep in mind, you are not only checking out 30k files into the working directory but svn creates a .svn subfolder with a local copy of each of these 30k files...
b) explorer: the explorer has quite a job to display folders with lots of items in it. it is time consuming to display all the stats in a list, create icons and what not.
i would suspect mainly problem a)