OK, got to my postgres. Here you are:
create or replace function explode_array(in_array anyarray) returns setof anyelement as
$$
select ($1)[s] from generate_series(1,array_upper($1, 1)) as s;
$$
language sql immutable;
SELECT s.* FROM score s
WHERE
s.id IN (
select
-- Get the high scoring score ID for each game:
explode_array(ARRAY(
-- Get the high score for game g:
SELECT
s2.id FROM score s2 WHERE s2.game_id =
g.id ORDER BY
s2.score DESC LIMIT 5
))
FROM game g
);
It takes ~64ms for me
Best regards, Vitaliy Tymchyshyn