Odd performance difference in check constraint : SQL(slow) vs plpgsql(fast)

This is the reverse of what I thought I would find.

In short my check constraint is extracting the epoch from a start timestamp,
and an end timestamp to get the number of seconds difference.
It then uses this number to check the array_upper() of an array to make sure
it's the proper size

The SQL version uses a case statement, and the plpgsql uses an IF/ELSE

In a particular insert test
The plpgsql version adds 1 second over the no constraints case.
the sql version adds 10 seconds over the no constraints case.

Why would this be?

--->

CREATE OR REPLACE FUNCTION check_end_time_foo( _start_time TIMESTAMP,
_end_time TIMESTAMP, _granularity SMALLINT, _values DOUBLE PRECISION[] )
RETURNS boolean AS $$
BEGIN
if( _granularity = 5 )
THEN
return( EXTRACT( EPOCH FROM _end_time at time zone 'utc' )::BIGINT -
EXTRACT( EPOCH FROM _start_time at time zone 'utc' )::BIGINT = 60 *
array_upper( _values,1 ) );
ELSEIF( _granularity = 7 )
THEN
return( EXTRACT( EPOCH FROM _end_time at time zone 'utc' )::BIGINT -
EXTRACT( EPOCH FROM _start_time at time zone 'utc' )::BIGINT = 900 *
array_upper( _values,1 ) );
ELSEIF( _granularity = 9 )
THEN
return( EXTRACT( EPOCH FROM _end_time at time zone 'utc' )::BIGINT -
EXTRACT( EPOCH FROM _start_time at time zone 'utc' )::BIGINT = 3600 *
array_upper( _values,1 ) );
ELSEIF( _granularity = 12 )
THEN
return( ( (EXTRACT( YEAR FROM (_end_time at time zone 'utc') at time
zone 'utc' )::INT * 12) + EXTRACT( MONTH FROM (_end_time at time zone 'utc'
) at time zone 'utc' )::INT ) - ( (EXTRACT( YEAR FROM (_start_time at time
zone 'utc') at time zone 'utc' )::INT * 12) + EXTRACT( MONTH FROM
(_start_time at time zone 'utc') at time zone 'utc' )::INT ) = array_upper(
_values,1 ) );
END IF;
END;
$$ language plpgsql IMMUTABLE;

alter table timeseries add CONSTRAINT timeseries_valid_end_time CHECK(
check_end_time_foo( series_start_time, series_end_time, granularity,
data_value ) );

-vs-

alter table timeseries add CONSTRAINT timeseries_valid_end_time CHECK( CASE
WHEN granularity = 5

THEN

EXTRACT( EPOCH FROM series_end_time at time zone 'utc' )::BIGINT - EXTRACT(
EPOCH FROM series_start_time at time zone 'utc' )::BIGINT = 60 *
array_upper( data_value,1 )

WHEN granularity = 7

THEN

EXTRACT( EPOCH FROM series_end_time at time zone 'utc' )::BIGINT - EXTRACT(
EPOCH FROM series_start_time at time zone 'utc' )::BIGINT = 900 *
array_upper( data_value,1 )

WHEN granularity = 9

THEN

EXTRACT( EPOCH FROM series_end_time at time zone 'utc' )::BIGINT - EXTRACT(
EPOCH FROM series_start_time at time zone 'utc' )::BIGINT = 3600 *
array_upper( data_value,1 )

WHEN granularity = 12

THEN

((EXTRACT( YEAR FROM ( series_end_time at time zone 'utc') at time zone
'utc' )::INT * 12) + EXTRACT( MONTH FROM ( series_end_time at time zone
'utc' ) at time zone 'utc' )::INT ) - ( (EXTRACT( YEAR FROM (
series_start_time at time zone 'utc') at time zone 'utc' )::INT * 12) +
EXTRACT( MONTH FROM ( series_start_time at time zone 'utc') at time zone
'utc' )::INT ) = array_upper( data_value,1 )

ELSE

false
END
);