How to fix iPhone X + iOS 13 Slow Wifi?

There is nothing to fix

What you measure are hardware and software differences.

The 2 iPhones you compare have a different internal density of components and different radio hardware. With higher density of components a lot of internal reflections occurs very near from the emitting source of radio frequency. Then the interferences caused by these reflections have a heavy impact on the signal / noise ratio of the radio signal.

To give you an image of what happens with short distance reflections just stick a large plane of plastic in front of a speaker and compare the sound quality with the speaker you left free.

Recommandations to make correct measures

If you would like to improve your comparison of hardware and software performance in the WI-Fi field, I would suggest to remove any internal source of load:

• Stop all automatic update:

  Settings > General > Software Update > Automatic Updates: Off

• Stop AirDrop:

  Settings > General > AirDrop: Receiving Off

• Stop Handoff:

  Settings > General > Handoff: Off

• Stop Wi-Fi calling:

  Settings > Phone > Wi-Fi Calling: Off

• Stop iTunes Wi-Fi Sync:

  On your Mac, iTunes disable the automatic syncing of your iPhones and the backup on Wi-Fi.