USB High Speed = USB 2.0 = 480mbit/sec
USB Full Speed = USB 1.x = 12mbit/sec
USB Low Speed = USB 1.x = 1.5mbit/sec
Note that these numbers do not indicate actual real-world performance (you will lose roughly 40% of it due to protocol overhead), just actual specification bandwidth/capacity.
As for what happens when you have different speed devices plugged into a single USB controller -- each device runs at their own separate independent speeds. Example scenario:
USB 2.0 controller has 4 ports. You have 3 devices plugged into it: one 480mbit device, one 12mbit device, and one 1.5mbit device.
Each device will operate at each of the negotiated speeds independent of one another. If you were to do constant transfers from each device simultaneously, you would find that you'd be getting, from each device respectively, roughly 280mbit/sec, 6mbit/sec, and 0.8mbit/sec. (The numbers are lower by ~40% due to protocol overhead, like I said).
This situation can change if you have a USB hub in the picture. Example: if you have a USB hub is a 2.0 hub, connected to a USB 1.x controller, you're only going to get USB 1.x speeds (in reality) from the devices hooked up to the USB 2.0 hub, despite the devices (on the hub) themselves being USB 2.0. In other words, the throughput limitation bottleneck is because you've got the 2.0 hub hooked up to a 1.x controller.
Finally, in some cases there are internal chip bandwidth limits, e.g. if you were to hook up 4 480mbit devices to the same controller and do transfers simultaneously would you get 280mbit/sec at each? Possibly, but then again in some cases you wouldn't (for example some of the Broadcom SOCs I've seen have internal bandwidth limits where effectively the entire USB 2.0 controller has a maximum bandwidth of 480mbit that's shared across all devices/ports, rather than 480mbit per device. So with two USB ports on that controller, operating at USB 2.0 speed, effectively each device would have 240mbit available to it, and in reality would get only 60% of that speed (due to protocol overhead)).
Hope this helps.--
Making life hard for others since 1977.
I speak for myself and not my employer/affiliates of my employer.