No, it is not related to the cart and its USB hub. It probably is the computer you're using to sync the devices that is causing the bottleneck. Here is a notice we recently posted to the web site:
"Syncing hardware update: Many new laptops, such as MacBook Air (purchased June 2013 or later) and MacBook Pro (purchased November 2013 or later), have hardware limitations as to the number of USB devices the laptop will recognize at any one time, similar to certain models of Windows notebook computers. This will limit the ability to sync more than 25 iPad devices via a wired connection at one time. This does not affect charging capability. Please see Product Bulletin 2013-6E
for more information. This is not a limitation of the Ergotron cart; check with your laptop manufacturer for details regarding USB device limits for your particular laptop model."