Long-running DELETE...WHERE...

Why would a delete involving a subselect run so much longer than the
individual delete commands?

My situation: table A has 200,000 rows. I've made up a temporary table
which holds the single-valued primary key for 80,000 rows which I want
to delete.
 DELETE FROM a WHERE id IN (select ID from tmp LIMIT 800);

runs for several minutes.  But if I do

\o tmpfile
\t
SELECT 'DELETE FROM a WHERE id = ' || id || ';' from tmp limit 800;
\o
\i tmpfile

this completes in about 15 seconds, or 1/50 of the time for the
single-statement delete above.

In trying to optimize this process, I disabled all the relational
integrity triggers (foreign keys) involving the table, and then I
dropped all the indexes EXCEPT that of the primary key.  All the
experiments were done within a single transaction using BEGIN. The
database version is 7.1.3, and the table was vacuumed very recently.