Curmudgeon wrote:
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?

Not without knowing how they are plumbed. Do they connect together under the
sink in the cabinet or do they enter the wall separately. Inside the wall
they are often connected to the vertical stack with a "cross" fitting,
something could be blocking one side and not the other, try snaking the line
out.