Video upload-php using ffmpeg transcoding mp4 time is too long

Find a FFmpeg support h264 encoding, PHP call exec take advantage of ffmpeg transcoding time too long, CPU utilization of more than 80%, ask how to solve.
transcoding command also do not know what the meaning of the surface parameters?

$command='..\ffmpeg\bin\ffmpeg.exe -i ' .  $file . '.' . $ext . ' -vcodec libx264 -vprofile main -preset slow -b:v 400k -maxrate 400k -bufsize 800k -vf scale=-1:480 -threads 0 -acodec libvo_aacenc -ab 128k ' .$file . '.mp4';

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Find a FFmpeg support h264 encoding, PHP call exec take advantage of ffmpeg transcoding time too long, CPU utilization of more than 80%, ask how to solve.
transcoding command also do not know what the meaning of the surface parameters?

$command='..\ffmpeg\bin\ffmpeg.exe -i ' .  $file . '.' . $ext . ' -vcodec libx264 -vprofile main -preset slow -b:v 400k -maxrate 400k -bufsize 800k -vf scale=-1:480 -threads 0 -acodec libvo_aacenc -ab 128k ' .$file . '.mp4';

Use seven bull to turn, or use the queue to do it asynchronously

Specific command man FFmpeg

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