I recently tried this with 4 gpus (3 cards) on one box. Fan query seems to be a bottleneck, at least for my setup (sdk 2.1)

If running getstatus.php takes more than 2 seconds for you, or you get occasional gpu at 0% reports (jitters in the charts, etc) you should try disabling fan monitoring.

Edit your config.json file and change "fan":"yes" lines to "fan":"no" and see if it changes anything for you.