You aren't going to like this...
As others have noted, this is a problem with USB 3.1 (formally 3.0), and one potentially infuriating downside to USB 3.1 that has largely driven me to abandon its usage almost entirely (and I think USB 2.0 type-C is going to be around for a very long time as an outcome of this.)
The 2016s seem more prone than the previous generation (which was certainly very prone as well, and even the SD slot could generate some issues in some instances since it was wired through the USB 3.0 bus) - but why does the 2016 seem more prone? Whether this is because of the design of the wifi card/wifi antenna, an outcome of the shielding Apple used (or lack of), an issue being heavily amplified by poorly shielded 3rd party devices (some of which Apple sells in their own store), an outcome of the increased signal clutter (which seems to amplify the issue), a combination, or none of the above, I cannot say. But I can say that even well-shielded cables still have this issue...although they can indeed make a huge improvement in functionality. Further, 5 GHz's own limitations of distance and barrier penetration means 2.4 GHz will be required for the foreseeable future, and for quite a few people it means 5 GHz exclusively may not be practical.
I agree with your assessment that removing the SD card was not a very good idea. If Apple left the slot in place, and ran it through the PCIe bus, giving it enough capacity to saturate cards that can transfer data at over 300 MB/s, that would have been spectacular and made a great computer even greater.
If you want to fully ELIMINATE the issue, you could...
- Switch your WiFI to 5GHz only (if your wifi signal has to travel through walls/floors, bear in mind 2.4 GHz does a far better job than 5 GHz at this), or
- switch over to a wired ethernet connection, or
- switch your SD card reader to something that uses USB 2.0 (or Thunderbolt 3 via a hub, provided the SD card slot in the hub is wired through TB) - USB 2.0 does NOT have this issue at all
- Use a wireless import via your camera, if it has one (this is painfully slow for me)
If you want to REDUCE the severity of the issue (but probably NOT fully eliminate it), you could...
- Switch the WiFi channel on your router - you will have to play with different channels as some do better than others, but a good place to start is trying whatever channels that macOS says is "optimal" for your machine
- Wrap the SD card reader (and cable, if it has one) in a shielding foil - even aluminum foil can help somewhat
- Relocate the physical position of the router closer to the computer (note, at some point, you will get close enough to make 5 GHz a possibility, and doing this will fully eliminate the issue)
- Use a USB 3.1 SD card reader that is physically far away (3-5+ feet) from the wifi antenna on the MBP, and use a USB-IF certified cable to connect the MBP to the card reader
- Switch to another brand SD card reader that has User reviews which confirm it has a less severe effect than others
- Switch to another brand router and/or look at signal amplification options
- Eliminate other sources of interference (ex: Bluetooth, other USB 3.1 (formally 3.0) cables with active signals, locations near microwaves/power lines, etc.
- (combining multiple of the above could have a compounding effect)
