[GENERAL] full vacuum really slows down query

I have a query that takes 2 sec if I run it from a freshly restored
dump. If I run a full vacuum on the database it then takes 30 seconds.

Would someone please comment as to why I would see a 15x slow down by
only vacuuming the DB? Reindexing does not help, and a full vacuum was
run just prior to the dump.

I run a full vacuum every night.

I am using 8.3.1

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