what cable is quicker to transfer files lightning to type c or lightning to usb 3.0?

FWIW. The cable will not be the limiting factor here. It would be the port types/data bus/CPUs on each device for the data transfer.

USB Type C defines the physical port dimensions; whereas, USB 1.0-4.0 defines the USB standards (in this case maximum data throughput). Thunderbolt 3&4 (standards) both use the USB-C type port, but have different data rate capabilities vs. USB.

equally slow I believe, the whole rumor stuff about the iphone15 having an usb-c connector but only the iphone15pro having a speed higher than usb2 speed, would suggest lightning bolt is also limited to usb2 speed, if not then it would be crazy to limit the non pro iphone 15 to that.