Slow insert

Hi guys,

Why would an INSERT ever be really slow? This is what I see a lot of in
our site logs:

Dec 5 15:57:48 marshall postgres[19599]: [3-1] LOG: duration:
13265.492 ms statement: INSERT INTO users_sessions (sid, cobrand_id,
uid) VALUES ('145982ac39e1d09fec99cc8a606155e7', '1', '0')

13 seconds to insert a single row!

It seems to happen at random times during the day. That sessions table
is heavily updated and inserted, and has pg_autovacuum running vacuum
analyze and analyze on it every few minutes I think.

We don't run any exclusive lock stuff on it.

So what lock or concurrency issue could cause a single-row insert to
take 13 seconds? Could vacuum analyze be doing it?

Thanks,

Chris