Query slow as function

Hello all!

I have a very simple query that I am trying to wrap into a function:

SELECT gs.geo_shape_id AS gid,
gs.geocode
FROM geo_shapes gs
WHERE gs.geocode = 'xyz'
AND geo_type = 1
GROUP BY gs.geography, gs.geo_shape_id, gs.geocode;

This query runs in about 10 milliseconds.

Now my goal is to wrap the query in a function:

I create a return type:
CREATE TYPE geocode_carrier_route_by_geocode_result AS
(gid integer,
geocode character varying(9));
ALTER TYPE geocode_carrier_route_by_geocode_result
OWNER TO root;

..and the function
CREATE OR REPLACE FUNCTION geocode_carrier_route_by_geocode(geo_code
character(9))
RETURNS SETOF geocode_carrier_route_by_geocode_result AS
$BODY$

BEGIN

RETURN QUERY EXECUTE
'SELECT gs.geo_shape_id AS gid,
gs.geocode
FROM geo_shapes gs
WHERE gs.geocode = $1
AND geo_type = 1
GROUP BY gs.geography, gs.geo_shape_id, gs.geocode'
USING geo_code;

END;

$BODY$
LANGUAGE plpgsql STABLE;
ALTER FUNCTION geocode_carrier_route_by_geocode(character)
OWNER TO root;

Execute the function: select * from geocode_carrier_route_by_geocode('xyz');

This query takes 500 milliseconds to run. My question of course is why?

Related: If I create a function and assign LANGUAGE 'sql', my function runs
in the expected 10 milliseconds. Is there some overhead to using the
plpgsql language?

Thanks for any help in clarifying my understanding!