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Apple to Limit iPhone 15 USB-C Cables to USB 2.0 Speeds: Report
(www.extremetech.com)
This is a most excellent place for technology news and articles.
The reason is that like with the iPhone 14, in the non-Pro models they put the SoC from the previous year's Pro model, and that one was only designed for Lightning so only USB 2.0. So the non-Pro will get USB 3 once the USB 3-supporting SoC trickles down from the Pro.