How to fix iPhone X + iOS 13 Slow Wifi? How do I fix slow wifi on an iPhone XR (iOS 13.4.1)?
The boss has an iPhone XR (iOS 13.4.1), and I have an iPhone 6 (iOS 12.4.5). The iPhone 6 gets 100% better Mbps throughput around my house. I have validated my router health and validated network setup on both phones. I have done all the reset/forget network cycling recommended by guides like this and this and this.
I measure the same performance percentage difference--iPhone 6 performs roughly 110% better--in all these tested scenarios:

802.11 b/g/n 2.4 GHz network connection on both phones, simultaneously
802.11 ac 5 GHz network connection on both phones, simultaneously (first screenshot)
Bluetooth off (second screenshot)

My test was:

hold both phones in hand
test average Mbps download speeds using Wi-Fi Sweetspots app
average over ~10 seconds
test 6 different spots around the house
record results

Here are the side-by-side results of the latter two tests; left side is iPhone XR + iOS 13.4.1, right side is iPhone 6 + iOS 12.4.5

Test with bluetooth radio off on both phones:

In both tests, &quot;Spot 4&quot; is standing directly next to the Wifi router.
Update:
Here are the recorded results in a Google Spreadsheet for at least 6 tests. The 5 of the top 6 results were for the iPhone 6 + iOS 12.4.5 combination.

Update 2:

both devices were tested without cases.
I have tested on four different routers: 2 models of Netgear, TPLink AV600, cheap-o ActionTek. Results were consistent across all 4 routers: iPhone 6 consistently had 100%+ better throughput.

Update 3:
My wifi was assigned to channels 7@2.4GHz and 44@5GHz, which my network analysis showed to be the only thing in the area on that channel. Also conducted a second test on alternate channels, with exclusive access; same results.
Update 4:
Both Phones were restarted occasionally, but not between every documented run. There was definitely a restart in the iPhoneX at least twice: once when the documented tests started, and once when I upgraded it from 13.3 to 13.4.1. Over the course of the few weeks that I have been observing the throughput metrics, the phones have been restarted ad-hoc and the slow relative behavior persists.
Update 5:
The following settings were in place at the time of the test:

Automatic Updates: Off
AirDrop: Receiving Off
Handoff: Off
Wi-Fi Calling: Off
iTunes Wi-Fi Sync: Off
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.