Extremely Slow Cascade Delete Operation

I have 3 tables - lot, unit and measurement

1 lot is having relationship to many unit.
1 unit is having relationship to many measurement.
delete cascade is being used among their relationship

I try to perform delete operation on single row of lot.
=======================================================
SemiconductorInspection=# select count(*) from lot;
count
-------
2
(1 row)

SemiconductorInspection=# select count(*) from unit;
count
---------
1151927
(1 row)

SemiconductorInspection=# select count(*) from measurement;
count
---------
9215416
(1 row)

SemiconductorInspection=# VACUUM ANALYZE;
VACUUM

SemiconductorInspection=# delete from lot where lot_id = 2;

Opps, this is a coffee operation. That's mean I can go out to have few cups of coffee and the operation still on going.

Even I use :
SemiconductorInspection=# EXPLAIN ANALYZE delete from lot where lot_id = 2;

It still hang there :(

Any suggestion? Anything I am doing wrong? Or this is the expected performance?

The table SQL is as follow :

IF NOT EXISTS(SELECT * FROM information_schema.tables WHERE table_name = 'lot') THEN
CREATE TABLE lot
(
lot_id bigserial NOT NULL,
CONSTRAINT pk_lot_id PRIMARY KEY (lot_id)
);
END IF;
IF NOT EXISTS(SELECT * FROM information_schema.tables WHERE table_name = 'unit') THEN
CREATE TABLE unit
(
unit_id bigserial NOT NULL,
fk_lot_id bigint NOT NULL,
CONSTRAINT pk_unit_id PRIMARY KEY (unit_id),
CONSTRAINT fk_lot_id FOREIGN KEY (fk_lot_id)
REFERENCES lot (lot_id) MATCH SIMPLE
ON UPDATE NO ACTION ON DELETE CASCADE
);
END IF;
IF NOT EXISTS(SELECT * FROM information_schema.tables WHERE table_name = 'measurement') THEN
CREATE TABLE measurement
(
measurement_id bigserial NOT NULL,
fk_unit_id bigint NOT NULL,
CONSTRAINT pk_measurement_id PRIMARY KEY (measurement_id),
CONSTRAINT fk_unit_id FOREIGN KEY (fk_unit_id)
REFERENCES unit (unit_id) MATCH SIMPLE
ON UPDATE NO ACTION ON DELETE CASCADE
);
END IF;

Thanks and Regards
Yan Cheng CHEOK