Why is PostgreSQL 9.2 slower than 9.1 in my tests?

I have upgraded from PostgreSQL 9.1.5 to 9.2.1:

"PostgreSQL 9.1.5 on x86_64-unknown-linux-gnu, compiled by gcc (GCC)
4.4.6 20120305 (Red Hat 4.4.6-4), 64-bit"
"PostgreSQL 9.2.1 on x86_64-unknown-linux-gnu, compiled by gcc (GCC)
4.4.6 20120305 (Red Hat 4.4.6-4), 64-bit"

It is on the same machine with default PostgreSQL configuration files (only
port was changed).

For testing purpose I have simple table:

CREATE TEMP TABLE test_table_md_speed(id serial primary key, n integer);

Which I test using function:

CREATE OR REPLACE FUNCTION TEST_DB_SPEED(cnt integer) RETURNS text AS $$
DECLARE
time_start timestamp;
time_stop timestamp;
time_total interval;
BEGIN
time_start := cast(timeofday() AS TIMESTAMP);
FOR i IN 1..cnt LOOP
INSERT INTO test_table_md_speed(n) VALUES (i);
END LOOP;
time_stop := cast(timeofday() AS TIMESTAMP);
time_total := time_stop-time_start;

RETURN extract (milliseconds from time_total);
END;
$$ LANGUAGE plpgsql;

And I call:

SELECT test_db_speed(1000000);

I see strange results. For PostgreSQL 9.1.5 I get "8254.769", and for 9.2.1
I get: "9022.219". This means that new version is slower. I cannot find why.

Any ideas why those results differ?

--
Patryk Sidzina