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

[bubba postgres <bubba.postgres@xxxxxxxxx>]

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 );