I have a double vanity in which the sink on the left drains very
slowly when water is first run. If I let the water drain out then run
the water again immediately the sink drains normally. The popup,
tailpiece and trap are all clear and operate normally; the sink on the
right does not exhibit this problem. This is unlikely to be a blockage
since the problem has been present since the house was first built;
also, if there were a blockage I don't think it would drain slowly one
minute and then freely when the water is run a second time.

Any ideas?