pg_dumpall affecting performance

Hello,

I was under the impression that pg_dumpall didn't affect database
performance when dumping while the db is live. However I have evidence to
the contrary now - queries that are run during the pg_dumpall time take 10
to a 100 times longer to execute than normal while pg_dumpall is running.
The strange thing is that this started after my database grew by about 25%
after a large influx of data due to user load. I'm wonder if there is a
tipping
point or a config setting I need to change now that the db is larger that
is
causing all this to happen.

Thanks,
  Mark