With AT&T fiber the iOS app for AT&T lets me run a system test that shows the up/download speed. The speeds are always inline or better than what I pay for. Just now the download speed is 1307Mbps and upload is 1213 Mbps. AT&T Internet 1000.

Wi-Fi from the AT&T router is never hitting anywhere near those numbers. Wi-Fi via the EERO Pro that is hard wired to the AT&T router gives me near what the app tells me the speeds are. The EERO itself reports similar up and download numbers as the AT&T router.

iPhone 16 Pro - Near top speed.
MS Surface Pro 9 - Near top speed.
2016 Dell all in one - Half the speed.

The dell only gets close if it's hardwired to the AT&T router.

If this is really important (meaning customer wants to pay you for the information/education) I would create a spreadsheet for him. Start with the ISPs app and test each device. Compare wired vs wireless for devices that have that can be hardwired.

If the client doesn't want to pay for it just respond with what you've already told them. A few sentences explaining how the Wi-Fi hardware/software in their devices is different. Distance from Access Points. Wired vs Wireless.

Note: I am listening to a webinar right now so some of this is unorganized rambling.