What if a USB 1.1 device is plugged into that hub?
If a USB 1.1 (Low-speed or Full-speed) device is plugged into USB2.0 hub, the slow signals for LS/FS devices are not flowing directly from PC host; they are TRANSLATED by the hub. Communication between PC and hub, however, occurs at HS rate (480 Mbps) using HS protocol. How it is done?
The LS/FS devices receive their corresponding traffic from the hub. USB 2.0 hubs all have special built-in communication processors called "Transaction Translators" (TT). The TT operates under a special class of HS traffic/commands called "split transactions". LS/FS transactions are "splt" into several parts on host's HS end. Short fast packets (at HS rate, "start split") are stored in the TT. The TT buffers the information, and then unfolds/transalates the command/data into lower speed protocol.
In very simplified terms, while the slow LS/FS data are in progress, the host "pings" TT with short packets to ensure the completion of slow downsream transactions, and "completes the split" when the low-end transaction finishes. This does not take much of bus time, so the hub's upstream bus is not that busy, and can service other HS devices or other TTs in other hubs, and several LS/FS devices at once. The split transaction protocol is fairly complicated: USB 2.0 specifications have about 150 pages explaining how this protocol should work.
That's why the 3-m limit does not apply to this smart topology, and the cable length can be at least 5m (see discussion of real cable limits here).