Vast wifi performance disparity between MacBook Pro and iPad/iPhone

If you wish to do a Deep dive on channel selection and spectrum, tools in Wireless Diagnostics (on that same Option-Wi-Fi menu) can show you the NOMINAL channel used by every Router in range.This is the SCAN display

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(drag and drop on Preview see larger)

Unfortunately, the channel numbers were assigned long ago, when the radios used only one channel. These days they sprawl across several to a dozen or more.

There is an inexpensive utility called WiFI Explorer that has a "Spectrum" display, and can give you a much more realistic picture:

(drag and drop on preview to see larger)

Note that is this display, the TWO Routers in the 5GHz band are DIRECTLY competing with each other, using all nominal channels between 36 and 50. These are 40 MHz each -- The US spectrum for 80 uses additional bandwidth that may be split.

Ok, I may be wrong here, but I thought I read somewhere that you should not use the right-side USB-C ports for the power connection ... as it could cause overheating issues ... which could affect the built-in AirPort card on the notebook. As such, I suggest trying switching to the left-side port for power to see if this helps.

The iPhone's 'claim to fame' is based on its radios. These are likely far more selective, even on Wi-Fi that your Mac. The iPad just borrows the great radios from the iPhone.

<<there are at least a dozen nearby (I'm in a condo building). Those don't seem to have an effect on the other devices, though.>>

These DO seem to be causing trouble on your Mac.

You might get different results by choosing a 40 MHz channel in your Router (rather than the 80 Mhz set currently) , and using "automatic" channel selection. "Automatic" channel selection means, when your Router first powers up, it samples the traffic on each channel, and chooses the channel with the least completion. it is not an accident that every debugging procedure start with "cycle the power OFF and ON again to your Router".

The use of a 40 MHz channel may give you a smaller footprint, and if lucky, less clobbering of your data by your neighbors' Routers.

But (based on signal levels) you appear to be Right on top of your Router. you could get faster speeds by connecting an ethernet cable when in that position.

Why do I have difficulty with Wi-Fi or Bluetooth devices when USB 3 devices are attached to my computer?

Some USB 3 devices can generate radio frequency interference that can cause Wi-Fi and Bluetooth devices operating in the 2.4GHz band to have issues communicating with your computer. Here are some tips to avoid this issue:

• If your USB device has a cable long enough that you can move the device, place it away from your Mac—and make sure not to place it behind your Mac, or near the hinge of its display. The antennas for Wi-Fi and Bluetooth are located there, and USB 3 devices placed there might interfere with your wireless connections.

• If you're using adapters or dongles on a Mac computer with Thunderbolt 3 (USB-C) ports, plug them into the front port on the left side of your Mac, or into the ports on the right side (if your computer has them). These ports are the farthest away from the antennas, making interference less likely.

• To avoid interference on the 2.4GHz band using Wi-Fi, try using the 5GHz band instead. You can change this on your wireless base station. Bluetooth always uses 2.4GHz, so this alternative isn't available for Bluetooth.

from:

About USB on Mac computers - Apple Support

Resolve Wi-Fi and Bluetooth issues caused by wireless interference

https://support.apple.com/en-us/HT201542

OK, lets have a look at that wi-Fi. Hold down the option key while you click on the Wi-Fi Icon on the menubar. you will get s snapshot of Current performance looks like this older one:

screenshot or transcribe the values in your snapshot,

and ALSO: How many Networks do you see?

You have made a connection in the 5GHz band at chapel 44. RSSI (signal) of EXTREMLY strong -37 less noise at -92 leaves signal-to-noise of 55, try excellent. You should be connecting at the absolute top speeds possible, ...

Using three antennas, top speed there would be 1300 M bits/sec.

Instead, you are using just barely more than 1 antenna, at 130 M Bits/sec, having dropped back from higher transmit speeds.