Insert statement taking a very long time to complete

I'm having some trouble with an SQL insert statement taking waaay too long
to run. It's been suggested that maybe I'm using inefficient SQL, and that there might be a faster way to achieve the results.

I have two tables with identical structure, and I want to copy all records
that exist in table A but not in table B, into table B.

Assuming that the structure is primary key "pk" and data fields "f1", "f2"
and "f3", here is the SQL statement I'm using:

  insert into tableb(pk, f1, f2, f3)
  (select * from tablea where tablea.pk not in (select pk from tableb))

This statement has worked well for small amounts of data, but when I tried
on a table with approximately 200k records, it took forever to finish.

A coworker with some SQL experience thinks that the "where" clause is being executed for *every* insertion. If so, this could certainly account
for much of the slowdown.

Anyone have any suggestions?

--
John Gordon                   A is for Amy, who fell down the stairs
gordon_at_panix.com              B is for Basil, assaulted by bears
                                -- Edward Gorey, "The Gashlycrumb
Tinies"