Optimizing LabelGrid - the result is worse than before??

OptimizeLabelGrid.sql

------- Testing:
-- git clone https://github.com/openmaptiles/openmaptiles
-- git checkout upgrade-v5-pg12
-- place this file in the openmaptiles/ dir as "test-func.sql"
-- use  make start-db   to create a new database (in docker)   
-- use  make bash       to start tools (another docker)
-- test with this command. The test call is taken from the  openmaptiles-tools/tests/sql/LabelGrid.sql  test. Note the "volatile" keyword - without it the query planner will optimize away multiple calls with the same value.
--    profile-pg-func --file test-func.sql "LabelGrid_pgsql(ST_GeomFromText('POINT(100 -100)',900913), 64*9.5546285343)" "LabelGrid_sql(ST_GeomFromText('POINT(100 -100)',900913), 64*9.5546285343)"

-- The results are not that great:
-- Function                                                                AVG of 8 runs    MIN             MAX                STDEV
-- ----------------------------------------------------------------------  ---------------  --------------  --------------  --------
-- LabelGrid_pgsql(ST_GeomFromText('POINT(100 -100)',900913), 64*9.5546285343)  0:00:01.060830   0:00:00.936732  0:00:01.235901  0.111131
-- LabelGrid_sql(ST_GeomFromText('POINT(100 -100)',900913), 64*9.5546285343)   0:00:02.010954   0:00:01.748449  0:00:02.192110  0.152105


create or replace function LabelGrid_pgsql (
    g geometry,
    grid_size numeric
)
    returns text
    language plpgsql volatile
    parallel safe as
$func$
begin
    if grid_size <= 0 then
        return 'null';
    end if;
    if GeometryType(g) <> 'POINT' then
        g := (select (ST_DumpPoints(g)).geom limit 1);
    end if;
    return ST_AsText(ST_SnapToGrid(
            g,
            grid_size/2,  -- x origin
            grid_size/2,  -- y origin
            grid_size,    -- x size
            grid_size     -- y size
        ));
end;
$func$;


create or replace function LabelGrid_sql (
    g geometry,
    grid_size numeric
)
    returns text
    language sql volatile
    parallel safe as
$func$
SELECT CASE
           WHEN grid_size <= 0 then 'null'
           ELSE ST_AsText(ST_SnapToGrid(
                   CASE
                       WHEN GeometryType(g) <> 'POINT' then (select (ST_DumpPoints(g)).geom limit 1)
                       ELSE g
                       END,
                   grid_size/2,  -- x origin
                   grid_size/2,  -- y origin
                   grid_size,    -- x size
                   grid_size     -- y size
               ))
           end;
$func$;