optimizing query performance

Hi! I've got the following statement:

SELECT DISTINCT sub.os,
COUNT(sub.os) as total
FROM (
SELECT split_part(system.name, ' ', 1) as os
FROM system, attacks
WHERE 1 = 1
AND timestamp >= 1205708400
AND timestamp <= 1206313200
AND attacks.source = system.ip_addr
AND NOT attacks.source IN (
SELECT exclusion
FROM org_excl
WHERE orgid=2
)
) as sub
GROUP BY sub.os
ORDER BY total DESC LIMIT 5

which has the following execution plan:

Limit (cost=1831417.45..1831417.48 rows=5 width=34) (actual time=
1599.915..1599.925 rows=3 loops=1)
-> Unique (cost=1831417.45..1831417.75 rows=41 width=34) (actualtime=
1599.912..1599.918 rows=3 loops=1)
-> Sort (cost=1831417.45..1831417.55 rows=41 width=34) (actual
time=1599.911..1599.913 rows=3 loops=1)
Sort Key: count(split_part(("system".name)::text, ''::text,
1)), split_part(("system".name)::text, ' '::text, 1)
-> HashAggregate (cost=1831415.63..1831416.35 rows=41
width=34) (actual time=1599.870..1599.876 rows=3 loops=1)
-> Nested Loop (cost=23.77..1829328.68 rows=417390
width=34) (actual time=0.075..1474.260 rows=75609 loops=1)
-> Index Scan using index_attacks_timestamp on
attacks (cost=23.77..2454.92 rows=36300 width=11) (actual time=
0.041..137.045 rows=72380 loops=1)
Index Cond: (("timestamp" >= 1205708400) AND
("timestamp" <= 1206313200))
Filter: (NOT (hashed subplan))
SubPlan
-> Seq Scan on org_excl
(cost=0.00..23.75rows=6 width=32) (actual time=
0.014..0.014 rows=0 loops=1)
Filter: (orgid = 2)
-> Index Scan using ip_addr_name_index on
"system" (cost=0.00..50.15 rows=12 width=45) (actual
time=0.009..0.012rows=1 loops=72380)
Index Cond: ("outer".source =
"system".ip_addr)

Total runtime: 1600.056 ms

the NL (nested loop) is accountable for most of the total query time. Is
there any way to avoid the NL and/or speed up the query?

Thanks,

Frits