un-understood index performance behaviour

Hi,
i have a table with a huge amount of rows (actually 4 millions and a half),
defined like this:

CREATE TABLE rtp_frame (
i_len integer NOT NULL,
i_file_offset bigint NOT NULL,
i_file_id integer NOT NULL, -- foreign key
i_timestamp bigint NOT NULL,
i_loop integer NOT NULL,
i_medium_id integer NOT NULL, -- foreign key
PRIMARY KEY(i_medium_id, i_loop, i_timestamp)
);

The primary key creates the btree index.

If I ask the database something like this:

SELECT ((max(i_timestamp) - min(i_timestamp))::double precision / <rate>)
FROM rtp_frame
WHERE i_medium_id = <medium> AND i_loop = <loop>;

it replies istantaneously.

But if i ask

DECLARE blablabla INSENSITIVE NO SCROLL CURSOR WITHOUT HOLD FOR
SELECT i_file_id, i_len, i_file_offset, i_timestamp
FROM rtp_frame WHERE i_medium_id = <medium>
AND i_loop = <loop>
AND i_timestamp BETWEEN 0 and 5400000
ORDER BY i_timestamp

on a medium with, say, 4 millions rows co-related, it takes 15 seconds to
reply, even with a different clause on i_timestamp (say i_timestamp >= 0),
even with the ORDER BY clause specified on the three indexed columns (ORDER
BY i_medium_id, i_loop, i_timestamp).

Issued on a medium with "just" some hundred thousand rows, it runs
instantaneously.

If I add a single btree index on i_timestamp, it runs instantaneously event
on a medium with millions rows (so having a btree(i_medium_id, i_loop,
i_timestamp) and btree(i_timestamp)).

With (btree(i_medium_id, i_loop) and btree(i_timestamp)), the first for sure
takes 15 seconds to run, the second i think too but not sure atm.

can anybody explain me why this happens ? and if i should try different
indexes ?

thanks a lot

Emiliano