Starting thunar takes very long, that means about 20 seconds. Even after showing up, it takes another 10 seconds until responding to pointer actions.
I have about 5 bookmarks to NFS and CIFS mounts. Those mounts are using autofs and mounted upon request. Starting thunar in Tree side pane mode opens all those mounts which is not necessary at all. The mounting itself is fast, the lookup of each bookmark may take some while. Anyway, starting Thunar does not need to scan all the bookmarks before using them.
Thanks for reporting!
.. .looks like your are not the first. Closing this one in favor of the existing bug. Seems to be related to CIFS.
*** This bug has been marked as a duplicate of bug 12467 ***
One thing which might help to figure out the cause:
Same bug if you only have nfs bookmarks ?