I have a query:

SELECT
SERIAL_NUM.SN,
PARTIDA_NAL.ELEMENT_ID,
IN_EL.SIMPLE_ID,
PARTIDA_NAL.IN_EL_ID
FROM
OPR INNER JOIN IN_EL ON (OPR.SKLAD_ID = :SKLAD AND IN_EL.OPR_ID = OPR.ID)
INNER JOIN PARTIDA_NAL ON IN_EL.ID = PARTIDA_NAL.IN_EL_ID
INNER JOIN SERIAL_NUM ON SERIAL_NUM.PARTIDA_EL_ID = PARTIDA_NAL.ELEMENT_ID
WHERE
PARTIDA_NAL.KOL > 0.00
ORDER BY 3

The results:
1 row is fetched and it takes 31 seconds for the first execution of this query.

CPU usage is between 0 and 4% with pauses with no CPU usage at all.
The joins are through foreign keys. OPR.SKLAD_ID is a foreign key and it has an index.

TABLES TOTAL ROWS READ
SERIAL_NUM 39687 39687 NON INDEXED READ
PARTIDA_NAL 440419 440419 INDEXED READ
OPR 322804 4979 INDEXED READ
IN_EL 367627 4979 INDEXED READ

1.171 s for the second execution.

And this is not a single event. I have a query that selects data from a selectable stored procedure.
In normal execution it takes about 40s with nearly 100% usage of the CPU's core.(some of the tables have over 1 mil. records)

But the first execution usually takes 5 to 10 minutes on my machine, again with very little CPU usage and the entire system works very slow until firebird return the results.

I moved the database to another machine, the results are similar.

What can be the problem? Is it something in Firebird or the problem is elsewhere?

My machine is:
Windows XP SP3
2GB RAM
AMD Athlon 64 X2 Dual 4000+ (2.1 GHz)
Firebird 2.1.2 Super server (with Classic server it is the same)