Wifi Reception iPhone 6s Vs XR
Typically the Wi-Fi antenna is kept close to the outer shell of the phone, so overall thickness shouldn't matter. This may be due to your wireless router being an older model and using an older standard than the XR was designed for (Wireless G/N, for example). Have you performed the speed test with both devices one after another, and have you attempted a test with a different Wi-Fi connection point to see if the problem is the router rather than the phone?