If you can live with limitations, piping your file through IJG's cjpeg, jpegtran and djpeg tools will be much faster than ImageMagick.

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$,="\n";foreach(split('',"\3\3\3c>\0>c\177cc\0~c~``\0cc\177cc")) {$a++;$_=unpack('B8',$_);tr,01,\40#,;$b[$a%6].=$_};print@b,"\n"