Using NOTIFY... Slow Client Querys

I'm using PostgreSQL 7.4.1. I have 140 clients connected on average
using libpq. When one client sends "NOTIFY timeclock;" to the server
all 140 clients are listening for it.

After receiving a notification from libpq (PQnotifies), each client
proceeds to execute a query for the last five records in the timeclock
table.

SELECT * FROM timeclock ORDER BY touched DESC LIMIT 5;

It varies, but it's often the case that clients wait up to 3 minutes
before the results come back. This seems like a really long time for a
query that I would think would go quickly. In fact, I can execute the
same query from a third party client and it runs fine, even while my
own client is still waiting for results.

Any ideas? It seems to be related to NOTIFY/LISTEN. Thanks!