So does each successive one become slower or how does that work?
No not a ton, the controller can handle a lot. Power is more of the problem, only has so much juice per slot. If those are all phone charger cables, possible hardly any of them actually charging a device.
I imagine power is split evenly. I wonder if speed is split evenly or successively slower.
Not really, it depends on what each device is doing. There is a shared aggregate bandwidth to draw from. It’s not a 50/50 type of thing