The techies here can explain this far better than I can. Two pass encoding gives better quality than single pass. Youíve chosen a high quality setting, a single pass encode will take about half the time. Check the encoding settings for VBR (variable bitrate) and CBR (constant bitrate).
Iím no whizz on any of this, but thereís a huge amount of info on it on the web Ė and lots of it on this forumís FAQ.