use strict;
use warnings;
use Time::HiRes 'time';
use Imager;

my $t = time;
my $i = Imager-> new( file => 'file.jpg' );
printf "image dimensions: %d x %d\n", 
    $i-> getwidth, $i-> getheight;
printf "reading: %.2f s\n", time - $t;

$t = time; $i-> copy
             -> scale( xpixels => 750, qtype => 'normal' )
             -> write( file => 'file+.jpg' );
printf "high quality resize & save: %.2f s\n", time - $t;
$t = time; $i-> copy
             -> scale( xpixels => 750, qtype => 'mixing' )
             -> write( file => 'file+.jpg' );
printf "medium quality resize & save: %.2f s\n", time - $t;
$t = time; $i-> copy
             -> scale( xpixels => 750 * 4, qtype => 'mixing' )
             -> scale( xpixels => 750, qtype => 'normal' )
             -> write( file => 'file+.jpg' );
printf "medium+ quality resize & save: %.2f s\n", time - $t;
$t = time; $i-> copy
             -> scale( xpixels => 750, qtype => 'preview' )
             -> write( file => 'file+.jpg' );
printf "low quality resize & save:\t\t %.2f s\n", time - $t;
$t = time; $i-> copy
             -> rotate( right => -90 )
             -> write( file => 'file+.jpg' );
printf "fast rotate & save: %.2f s\n", time - $t;
$t = time; $i-> copy
             -> rotate( degrees => -90 )
             -> write( file => 'file+.jpg' );
printf "slow rotate & save: %.2f s\n", time - $t;

__END__

image dimensions: 6083 x 4636
reading: 1.23 s
high quality resize & save: 6.52 s
medium quality resize & save: 1.17 s
medium+ quality resize & save: 3.84 s
low quality resize & save: 0.58 s
fast rotate & save: 3.41 s
slow rotate & save: 19.33 s
[download]

Some options were introduced for a reason... + Use medium size image as source for thumb (not shown above).

Updated: GD is so much faster, what do you mean "messed with colors"? Should work fine for geometric transformations.

use strict;
use warnings;
use Time::HiRes 'time';
use GD;

my $t = time;
my $i = GD::Image-> newFromJpeg( 'file.jpg', 1 );
printf "image dimensions: %d x %d\n", $i-> getBounds;
printf "reading: %.2f s\n", time - $t;

for ( 1 .. 20, 22, 23 ) {
    $t = time; 
    $i-> interpolationMethod( $_ );
    $i-> copyScaleInterpolated( 750, 572 )
      -> _file( 'file+.jpg' );
    printf "resize method \"$_\" & save: %.2f s\n", time - $t;
}

$t = time;
$i-> copyRotateInterpolated( 90, 0 )-> _file( 'file+.jpg' );
printf "rotate & save: %.2f s\n", time - $t;

__END__
 
image dimensions: 6083 x 4636
reading: 1.47 s
resize method "1" & save: 0.59 s
resize method "2" & save: 0.62 s
resize method "3" & save: 0.07 s
resize method "4" & save: 0.25 s
resize method "5" & save: 0.24 s
resize method "6" & save: 0.55 s
resize method "7" & save: 0.53 s
resize method "8" & save: 0.55 s
resize method "9" & save: 0.55 s
resize method "10" & save: 0.55 s
resize method "11" & save: 0.53 s
resize method "12" & save: 0.55 s
resize method "13" & save: 0.55 s
resize method "14" & save: 0.55 s
resize method "15" & save: 0.53 s
resize method "16" & save: 0.04 s
resize method "17" & save: 0.54 s
resize method "18" & save: 0.58 s
resize method "19" & save: 0.62 s
resize method "20" & save: 0.59 s
resize method "22" & save: 0.09 s
resize method "23" & save: 0.12 s
rotate & save: 3.03 s
[download]

Interpolation constants here. 21st died on me.