Performance issue: jsonb_object_agg() is twice slower than to_jsonb()

Hello,

while testing a best-performance implementation of my code I've noticed
very strange performance issue - jsonb_object_agg() is twice slower
than to_jsonb(select...).

Here are the results:

"PostgreSQL 9.5.4, compiled by Visual C++ build 1800, 64-bit"
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
- - - - - - - -

jsonb_object_agg -> 5.9 sec
to_jsonb -> 3.7 sec

PostgreSQL 9.6rc1, compiled by Visual C++ build 1800, 64-bit
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
- - - - - - - -

jsonb_object_agg -> 6.0 sec
to_jsonb -> 3.2 sec

Here is the code i've used to test performance. To my mind
"jsonb_object_agg' should be twice faster because it does not introduce
temp rowsets to be converted to jsonb. However actual result is the
opposite.
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
- - - - - - - -
DO LANGUAGE plpgsql $$
DECLARE
jsonb_result jsonb;
count int;
BEGIN
count = 0;
LOOP
-- Impl #1
-- jsonb_result = jsonb_object_agg('created', now() );

-- Impl #2
select to_jsonb(t) from (select now() as "created") t
into jsonb_result;

count = count + 1;
EXIT WHEN count > 500000;
END LOOP;

raise notice 'result = %', jsonb_result;
END; $$