DELETE running at snail-speed

I'm running a delete command on a postgresql-8.1.9 server. 25000
tuples are delete, time is ~ 400 secs

the command:
delete from downtime where start<'Aug 1 00:00:00 2008'


The table definitions are:
linie4=> \d downtime
                                         Table "public.downtime"
   Column   |            Type             |
Modifiers
------------+-----------------------------
+---------------------------------------------------------------
 downtimeid | integer                     | not null default nextval
('downtime_downtimeid_seq'::regclass)
 status     | smallint                    |
 start      | timestamp without time zone | default now()
 machineid  | smallint                    |
 a_nr       | integer                     |
Indexes:
    "downtime_pkey" PRIMARY KEY, btree (downtimeid)
    "idx_downtime_start" btree ("start")
Foreign-key constraints:
    "machineid_ok" FOREIGN KEY (machineid) REFERENCES machine
(machineid) ON UPDATE CASCADE ON DELETE CASCADE

linie4=> \d downtime_detail
                               Table "public.downtime_detail"
   Column   |   Type   |
Modifiers
------------+----------
+--------------------------------------------------------------------
 detailid   | integer  | not null default nextval
('downtime_detail_detailid_seq'::regclass)
 downtimeid | integer  |
 detail     | smallint |
Indexes:
    "downtime_detail_pkey" PRIMARY KEY, btree (detailid)
Foreign-key constraints:
    "statusid_ok" FOREIGN KEY (downtimeid) REFERENCES downtime
(downtimeid) ON UPDATE CASCADE ON DELETE CASCADE

I suspect the foreign key constraint of downtime_detail to slow down
the delete process. Is this a bug, probably fixed in latest version
(8.1.x) or should I drop the constraint and recreate after deletion -
which I only see as workaround ?

Gerhard