Postgres's Performance degrades after heavy db operation

Hi,

I am using Postgresql 8.1.2 and performing quite a huge number of db
operations ( insert/update/ delete) in around 4 tables. Around 3 million
of db is inserted and then 600 K rows are updated for around 5-6 times
during a day. Its observed that running this process for few days ( 5-6
days) makes the db access quite slower. Auto-Vacuum is set to ON with
other parameters set to default.

Slony 1.1.5 is being used for the db replication simultaneously.

Any ideas/pointers on what could be making the DB slow after all these
heavy operations being performed?

Thanks

Preeti