iClarified have posted a new Wi-Fi speed test comparing the iPhone XS Max, iPhone XS, and iPhone X. All three devices are running iOS 12 and connected to an Apple AirPort Extreme with no other devices connected to the wireless network.
Using a custom built application, we measured the speed at which each device was able to download three files 10MB, 50MB, and 100MB in size. The files were hosted on a Mac mini connected via an Ethernet cable to the AirPort Extreme.
iFixit’s teardowns revealed that the iPhone X has a Apple/Murata USI 170821 339S00397 WiFi / Bluetooth module, the iPhone XS has an Apple/USI 339S00551 SoC and the iPhone XS Max has a 338S00540 WiFi/Bluetooth SoC.
On its specifications page, Apple notes that the iPhone X supports ‘802.11ac Wi‑Fi with MIMO’; whereas, the iPhone XS supports ‘802.11ac Wi‑Fi with 2×2 MIMO’.