Low speed XBOX series X on LAN port RBK53

Thanks for posting the screen shot.  As FURRYe38 remarked, the Orbi 'app' reports the results that are conducted on the Orbi router.  It does not perform a speed test itself. There is a connection between the router and the device running the app which is used to communicate everything that appears on the app, including the Speed Test results.  Actual performance over WiFi can be measured by downloading a speed test app to the smartphone/tablet or by using a web browser on the smartphone/tablet to connect to one of the speed testing web sites, such as Ookla Speed Test (https://www.speedtest.net ) or Google Speed Test (https://fiber.google.com/speedtest/ )

Another unfortunate aspect of the RBK53 product is that Netgear released new firmware for the product in February, 2023, but the Firmware Update feature does not report it or offer to install it.  (No one knows why.)  I doubt very much that the specific firmware version has any effect on the speed of wired devices.  However, one of the common recommendations when discussing electronic products is, "update the software".  If you should want to look into this, the newer firmware can be found here: https://kb.netgear.com/000065539/RBR50-RBS50-Firmware-Version-2-7-5-4 

Manual firmware updates are a bit tricky.  Instructions can be found here:

https://kb.netgear.com/31573/How-do-I-manually-upgrade-firmware-on-my-Orbi-router-using-orbilogin-com 

When testing speed with the XBox, is the same cable used for the connection?

Have you tried using different LAN ports on the Orbi router to perform the speed tests?