This post[1] says > What the problems is - how the USB-C lines within a cable are used is determined at plug-in time, depending on what you plug in downstream of the port. For example, you can't have full speed USB 3.1 and full video at the same time. So the USB-C makes a choice for you. If you plug in both a high-res monitior and USB 3, then you can't have both at once.
Still not technically clear to me. It does sound like with just a hub your transfer speed will drop when using a hub and driving a monitor. Maybe the limitation I'm thinking of is if USB-C is carrying something like Thunderbolt, you can't carry a second Thunderbolt signal? Because USB-C splitting compromises so much, they're so rare in the market?
I have heard the order in which you plug in USB-C will tell it which direction to charge (which gives credence to things being determined at plug-in time). For example, plugging the cable into your phone then plugging it into a computer will make the phone charge the computer instead of vice versa. When I heard that they were speaking about a specific phone and laptop, so it wasn't theoretical, but I'm having a hard time corroborating this. This [2] article says the direction power flows is a computer setting. What's scary is there's a lot of articles warning you not to use your USB-C laptop charger to charge your phone because it could fry it.
[1] https://apple.stackexchange.com/questions/206365/why-are-the...
[2] https://www.digitaltrends.com/computing/charging-via-usb-c/
I'm not sure how all those one-cable USB-C docking stations work then, as they carry video and audio one direction (to the dock and then out to the screen), and multiple USB devices and power the other direction (from the wall and connected peripherals and through the dock). If it truly couldn't do multiple things at once, none of these docking stations would work with a single cable, and they do.