I got this from a Raspberry PI forum:
Single-TT hubs perform very badly with multiple USB1.1 devices plugged in.
Hubs have a mechanism to separate low- and full-speed devices from USB2.0 high-speed devices. This is called a transaction translator - which is responsible for performing the USB1.1-speed transactions. There are two options for implementing a hub - one with a single TT for all ports (the “cheap” option), and one TT per port. If multiple USB1.1 devices are plugged into a hub with a single TT, then all these devices must be talked to through this TT. The FIQ enforces a one-at-a-time rule whan talking to a device behind a TT. Exclusive access to a transaction translator means that for hubs with only 1 TT there is a limit on the number of devices that can be used at any one time, or scheduling conflicts cause lost packets. USB audio is particularly vulnerable when sharing a TT with other USB1.1 devices. The use of multi-TT hubs is strongly recommended - you can get this information from a lsusb -v | less and scrolling through to the listing for the external hub.