The USB-C port on the iPhone 15 (non-Pro) is limited to 480Mbps. Theoretically this is the same as the lightning port speeds. This is because the non-Pro 15 models are running the A16 processor and controller.

iPhone 15 Pro/Pro Max can support transfer speeds up to 10Gbps. But, you need the right cable. The one that comes with the phone in the box, and most USB-C charging cables, only support USB2.0 data speeds (480Mbps). You need a USB 3.1 Gen2 certified cable to get the full transfer speed out of the 15 Pro/Pro Max port.

Note that such cables tend to be kinda short… usually around 3 feet max.

Will it make a difference? That really depends on your wireless carrier. In some isolated cases I can exceed 480Mbps on Verizon and AT&T when I can get ultra Wideband 5G. Most of the the time I’m not hitting that limit though. And that’s on-device speeds. Tethered (hotspot) speeds are always de-prioritized.