DB Slowing Down

Hi,
we have a database. not to big about 50 tables, 20m records. On a daily
basis we update/insert/delete between 500k to 1M records in total. We
run a full vacuum every night. The db grows probably by 200k records
each day and at the end of the month it gets cleaned up.
We run 7.4.1 and 7.4.6

Now, after a few weeks the db starts to slow down after about 2 months
same operations take 2-3 times as long as originally. The only way to
speed it up again is to drop the entire database; run an initdb, create
and restore the database.

Is that an expected behavior?
Any help or suggestions are appreciated.

Thanks
Alex