postgres function update large table and deletion performance - postgresql

Question

postgres version: 9.3
postgres.conf: all default configurations
I have 2 tables, A and B,both have 1 million rows.
There is a postgres function that will execute every 2 seconds, it will update Table A where ids in an array(array size = 20), and then delete the rows in Table B.
DB function shows as below:
CREATE OR REPLACE FUNCTION test_function (ids NUMERIC[])
RETURNS void AS $$
BEGIN
UPDATE A a
SET status = 'begin', end_time = (NOW() AT TIME ZONE 'UTC')
WHERE a.id = ANY (ids);
DELETE FROM B b
WHERE b.aid = ANY (ids)
AND b.status = 'end';
END;
$$ LANGUAGE plpgsql;
Analysis shows as below:
explain(ANALYZE,BUFFERS,VERBOSE) select test_function('{2,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20}');
QUERY PLAN
Result (cost=0.00..0.26 rows=1 width=0) (actual time=14030.435..14030.436 rows=1 loops=1)
Output: test_function('{2,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20}'::numeric[])
Buffers: shared hit=24297 read=26137 dirtied=20
Total runtime: 14030.444 ms
(4 rows)
My Question is:
In the production environment, why this function need to execute at most 7 seconds before success;
When this function is executing, this process will eats up to 60%. --> This is the key problem
EDIT:
Analyze each single sql:
explain(ANALYZE,VERBOSE,BUFFERS) UPDATE A a SET status = 'begin',
end_time = (now()) WHERE a.id = ANY
('{2,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20}');
QUERY PLAN
Update on public.A a (cost=0.45..99.31 rows=20 width=143) (actual time=1.206..1.206 rows=0 loops=1)
Buffers: shared hit=206 read=27 dirtied=30
-> Index Scan using A_pkey on public.a a (cost=0.45..99.31 rows=20 width=143) (actual time=0.019..0.116 rows=19 loops=1)
Output: id, start_time, now(), 'begin'::character varying(255), xxxx... ctid
Index Cond: (t.id = ANY('{2,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20}'::integer[]))
Buffers: shared hit=75 read=11
Trigger test_trigger: time=5227.111 calls=1
Total runtime: 5228.357 ms
(8 rows)
explain(ANALYZE,BUFFERS,VERBOSE) DELETE FROM
B b WHERE tq.aid = ANY
('{2,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20}');
QUERY PLAN
Delete on B b (cost=0.00..1239.11 rows=20 width=6) (actual time=6.013..6.013 rows=0 loops=1)
Buffers: shared hit=448
-> Seq Scan on B b (cost=0.00..1239.11 rows=20 width=6) (actual time=6.011..6.011 rows=0 loops=1)
Output: ctid
Filter: (b.aid = ANY ('{2,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20}'::bigint[]))
Rows Removed by Filter: 21743
Buffers: shared hit=448
Total runtime: 6.029 ms
(8 rows)
CPU usage
Before calling:
Afer frequently operations: