How can this be?

Hello all,
Mostly Postgres makes sense to me. But now and then it does something
that boggles my brain. Take the statements below. I have a table
(agent) with 5300 rows. The primary key is agent_id. I can do SELECT
agent_id FROM agent and it returns all PK values in less than half a
second (dual Opteron box, 4G ram, SATA Raid 10 drive system).

But when I do a DELETE on two rows with an IN statement, using the primary
key index (as stated by EXPLAIN) it take almost 4 minutes.
pg_stat_activity shows nine other connections, all idle.

If someone can explain this to me it will help restore my general faith in
order and consistancy in the universe.

Martin

-- Executing query:
SELECT count(*) from agent;
Total query runtime: 54 ms.
Data retrieval runtime: 31 ms.
1 rows retrieved.
Result: 5353

-- Executing query:
VACUUM ANALYZE agent;

-- Executing query:
DELETE FROM agent WHERE agent_id IN (15395, 15394);
Query returned successfully: 2 rows affected, 224092 ms execution time.

-- Executing query:
EXPLAIN DELETE FROM agent WHERE agent_id IN (15395, 15394);
Index Scan using agent2_pkey, agent2_pkey on agent (cost=0.00..7.27
rows=2 width=6)
Index Cond: ((agent_id = 15395) OR (agent_id = 15394))

Here's my table
CREATE TABLE agent
(
agent_id int4 NOT NULL DEFAULT nextval('agent_id_seq'::text),
office_id int4 NOT NULL,
lastname varchar(25),
firstname varchar(25),
...other columns...
CONSTRAINT agent2_pkey PRIMARY KEY (agent_id),
CONSTRAINT agent_office_fk FOREIGN KEY (office_id) REFERENCES office (office_id) ON UPDATE RESTRICT ON DELETE RESTRICT
)
WITHOUT OIDS;