When Windows indicates that using a USB 3.0 port for your iPhone 15 provides better performance than a USB-4 port, it likely means that the iPhone 15's USB-C port, even though it's USB-C, is not fully utilizing the capabilities of the USB-4 port and is instead limited to USB 2.0 speeds. 

Here's a breakdown:
USB-4 vs. USB 3.0:
USB-4 is a newer standard that can handle much faster data transfer rates than USB 3.0. However, the iPhone 15's USB-C port, at least in the standard model, is only capable of USB 2.0 speeds (480 Mbps). 
iPhone 15 USB-C Limitations:
The iPhone 15's USB-C port, while a standard connector, is not designed to take full advantage of the speed of USB-4 or even USB 3.0. 
Why Windows Suggests USB 3.0:
Windows might be suggesting USB 3.0 because it recognizes that the iPhone 15 is not utilizing the full potential of the USB-4 port and may be able to achieve better performance (though still limited) by using a USB 3.0 port. 
iPhone 15 Pro/Pro Max:
The iPhone 15 Pro and Pro Max models do support USB 3.2 Gen 2 speeds (up to 10 Gbps), but you'll need a compatible USB-C cable to take advantage of it. 
In short:
The iPhone 15 standard model's USB-C port is limited to USB 2.0 speeds, so even though you might be using a USB-4 port, the performance will be no better than a USB 3.0 port, and Windows may even suggest that USB 3.0 is better because it's recognizing the limitations of the iPhone's USB-C port. 

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I have an iPhone 15 Pro which has more capability (allegedly)

My USB A ports are 5 gigabit when they should be 20 gigabit to support USB 3.2