slow, long-running 'commit prepared'

I have a pg instance with 700GB of data, almost all of which is in one
table. When I PREPARE and then COMMIT PREPARED a transaction that
reads & writes to a large fraction of that data (about 10%,
effectively randomly chosen rows and so every file in the table is
modified), the COMMIT PREPARED sometimes takes a very long time--2 to
5 minutes. Is this expected? Is it possible for the commit to hang
waiting on some lock for this long? I haven't yet been able to examine
pg_locks during this 5 minute delay. There is very little
concurrency--only a couple of open sessions--when the COMMIT PREPARED
is issued.

Thanks for your help,
John