my experience is only on OS X where i ran between 4 to 9 usb logitech c920 cams at half HD.
placing each videograbber in a separate thread made the overall performance much better.
for the 9 usb cam app i first used a recent macbook pro but later switched to a mac pro. again this improved the apps fps.
it didn't so much improve the fps of each camera but the fps of the main app.
first i ran each cam on full HD which made each cam serve images at 12 fps. going to half HD gave me double that fps.
using more than 3 USB cams on the macbook pro required the use of a this thunderbolt hub:
https://eshop.macsales.com/preorder/OWC-Thunderbolt2-Dock/
for the 9 cam app i used two of these hubs.
good luck.
