Slow query: table iteration (8.3)

Hitting a performance issues that I'm not sure how to diagnose.

SELECT highscores_for_steps_and_card(s.id, 591, 1) FROM stomp_steps s;
Seq Scan on stomp_steps s (cost=0.00..793.52 rows=2902 width=4)
(actual time=26509.919..26509.919 rows=0 loops=1)
Total runtime: 26509.972 ms

The inner function looks like this:

CREATE FUNCTION highscores_for_steps_and_card(steps_id int, card_id
int, count int) RETURNS SETOF INTEGER LANGUAGE SQL AS $$
SELECT r.id FROM stomp_round r
WHERE ($1 IS NULL OR r.steps_id = $1) AND ($2 IS NULL OR
r.user_card_id = $2)
ORDER BY r.score DESC LIMIT $3
$$

Limit (cost=13.12..13.12 rows=1 width=8) (actual time=0.054..0.054
rows=0 loops=1)
-> Sort (cost=13.12..13.12 rows=1 width=8) (actual
time=0.051..0.051 rows=0 loops=1)
Sort Key: score
Sort Method: quicksort Memory: 17kB
-> Bitmap Heap Scan on stomp_round r (cost=9.09..13.11
rows=1 width=8) (actual time=0.036..0.036 rows=0 loops=1)
Recheck Cond: ((280 = steps_id) AND (user_card_id = 591))
-> BitmapAnd (cost=9.09..9.09 rows=1 width=0) (actual
time=0.032..0.032 rows=0 loops=1)
-> Bitmap Index Scan on stomp_round_steps_id
(cost=0.00..4.40 rows=20 width=0) (actual time=0.030..0.030 rows=0
loops=1)
Index Cond: (280 = steps_id)
-> Bitmap Index Scan on stomp_round_user_card_id
(cost=0.00..4.44 rows=25 width=0) (never executed)
Index Cond: (user_card_id = 591)
Total runtime: 0.153 ms
(12 rows)

stomp_steps has about 1500 rows, so it finds 1500 high scores, one for
each stage.

I expected scalability issues from this on a regular drive, since
it'll be doing a ton of index seeking when not working out of cache,
so I expected to need to change to an SSD at some point (when it no
longer easily fits in cache). However, I/O doesn't seem to be the
bottleneck yet. If I run it several times, it consistently takes 26
seconds. The entire database is in OS cache (find | xargs cat:
250ms).

I'm not sure why the full query (26s) is orders of magnitude slower
than 1500*0.150ms (225ms). It's not a very complex query, and I'd
hope it's not being re-planned every iteration through the loop. Any
thoughts? Using SELECT to iterate over a table like this is very
useful (and I don't know any practical alternative), but it's
difficult to profile since it doesn't play nice with EXPLAIN ANALYZE.

--
Glenn Maynard