Postgres and "autocommit"

I turned autocommit on for our Django project and our unittests fail
unless we put "transaction.rollback" in our tearDown. Any idea why?
After fixing that I also noticed that our unittests are *much* slower.
Is it because they're mainly doing deletes and writes (we also load in
fixtures via YAML) and they don't get accumulated before being written
to disk? Thanks!

Abhik