Reading an article on Linux Gazette and reporting his own experience Jan Engelhardt asked on the lkml why reiserfs has the longest mount time among the filesystems that were tested. Jeffrey Mahoney explained:

ReiserFS caches bitmaps on mount and
for large file systems it can take a quite a long time. I've heard
reports of up to 15 minutes on multi-TB file systems

But there are patches that solve this issue.