The iPhone X is rated for 802.1ac performance, so in theory it could handle up to a 1300 Mbps connection.
Cannot answer for Apple on a user to user forum.....(we are all users here).......but for what it's worth, my iPhone X averages about 260-265 Mbps on a 300 Mbps Internet connection with the iPhone located in the next room from the WiFi router. 280-285 when the iPhone is in the same room with line of sight to the Wi-Fi router.
My wife's iPhone X does a little better....about 265-270 Mbps at the same general location a room away from the router.
My Mac laptop....about 7 years old.....comes in at about 285-290 Mbps on several checks.