Does an INTERNAL USB hub affect performance?

Will the performance of a USB hub connected directly to the motherboard be shared between the devices plugged into the hub?

The hub is basically invisible. The hub's uplink ports still only have however much bandwidth they have, and that bandwidth can be used by the host to access any of the downlink ports.

How is the power AND data speed distributed in such a hub?

That's completely up to the host. The total power to all the downlink ports can't exceed the power available from the uplink port, since this hub has no separate power source. And the total speed of all downlink ports can't exceed the speed of the uplink ports. But how that's divided is completely up to the software on the host, the hub has no say.