Strong slowdown on huge tables

Hello,

i use PostgreSQL 8.4.8 on Centos 5.x and i have some table where i load with pg_bulkload webtraffic logs, every day.
After loading new data, i delete with a query 30-days older logs.
Autovacuum is on.

In some systems, i experience a very huge slowdown of this table in queries, expecially update and delete; the database size grows up a lots.
I tried to rebuild indexes and make a manual vacuum; database size reduces, but slowdown problems remains.

Are there particular things to know after using pg_bulkload?
In this case, is preferred to set autovacuum off and launch one time a week, as example, the autovacuum full manually?

Thank you!
Francesco