PL/pgSQL functions - text / varchar - havy performance issue?!

Hi,

I'm using PostgreSQL 7.3.4 and noticed a havy performance issue when
using the datatype text for PL/pgSQL functions instead of varchar.

This is the table:

CREATE TABLE user_login_table (
id serial,
username varchar(100),
PRIMARY ID (id),
UNIQUE (username)
);

This table contains ~ 500.000 records. The database runs on a P4 with
512 MB RAM. When using the following functions, I notice a havy
speed difference:

CREATE OR REPLACE FUNCTION get_foo_exists (varchar(100))
RETURNS bool
AS '
BEGIN
PERFORM username
FROM user_login_table
WHERE username = $1;

RETURN FOUND;
END;
'
LANGUAGE 'plpgsql';

CREATE OR REPLACE FUNCTION get_foo_exists2 (text)
RETURNS bool
AS '
BEGIN
PERFORM username
FROM user_login_table
WHERE username = $1;

RETURN FOUND;
END;
'
LANGUAGE 'plpgsql';

The function 'get_foo_exists (varchar(100))' is extremly fast
(can't estimate - < 0.5 seconds). The function 'get_foo_exists2 (text)'
takes about 3 seconds for the same operation.
Is that normal?

Bye,
Oliver