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 &gt; General &gt; Software Update &gt; Automatic Updates: Off
Stop AirDrop:

Settings &gt; General &gt; AirDrop: Receiving Off
Stop Handoff:

Settings &gt; General &gt; Handoff: Off
Stop Wi-Fi calling:

Settings &gt; Phone &gt; 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.