UNIX internals question

[utzoo!decvax!ucbvax!ihnss!mhtsa!harpo!npois!houxi!houxd!houxc!houxb!houxa!houxe!lime!we13!rjk]

On internal UNIX 3.0 & 4.0 the first write to a newly mounted filesystem
causes a delay of 10-20 seconds, depending on system load.  This may be
happening on other versions and vintages too.  What gives here?  Surely
it can't take that long for an iget to return block pointers and the
like!  Is there some free-block verification going on?  Is each inode
being tested for allocation?  For my own curiousity, I would like to know
the scenario going on here.