Optimize query: time of "single * IN(many)" > time of "many * IN(single)"

Hello!

Can anyone help me out with the following situation:
(a) a single query with 550 id's in the IN-clause resulting into 800+
seconds;
(b) 550 queries with a single id in the IN-clause resulting into overall
time of <60 seconds;
The table consists of 950.000 records, and the resultset consists of 205.000
records.

>> Why is there such an extreme difference in time?
>> And is there a way to reduce the difference in time?
More information about the situation is below.

Thank you for your help and time! =)

Postgres-version
7.3.1

The query is like:
SELECT a_id, b_id, score, c_id, d_id
FROM tbl_scores
WHERE a_id IN(...)
UNION
SELECT a_id, b_id, score, c_id, d_id
FROM tbl_scores_alike
WHERE a_id IN(...)

Definition of tables is like:
___Fields
CREATE TABLE public.tbl_scores (
id int4 DEFAULT nextval('"tbl_scores_id_seq"'::text) NOT NULL,
a_id int4,
b_id int4,
score int4, d_id int8,
CONSTRAINT tbl_scores_pkey UNIQUE (id),
dc date DEFAULT now(),
c_id int4,
INITIALLY IMMEDIATE
) WITHOUT OIDS;
___Index
CREATE INDEX tbl_scores_idx10 ON tbl_scores USING btree (a_id, b_id,
score, c_id, d_id);

Things that I tried to reduce the time of situation (a) - single * IN(many):
* vacuum of the database; hardly any improvement.
* selecting a single field in the resultset (a_id) instead of all fields;
hardly any improvement.
* only querying one table, skipping the UNION; hardly any improvement;
* ... what would you try?

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