Hey Dave!
...What does work fine is the command line Python script,
wget -O speedtest-cli https://raw.githubusercontent.com/sivel/speedtest-cli/master/speedtest.py
python speedtest-cli
You may want to take the browser out of the equation or at least do tests with the Python script and compare.
Thanks for the reminder...I did actually use this in the past, as per your earlier suggestion from a year ago, and now i went back to re-test using that process as well. I picked the same server ID to keep the results somewhat consistent with what i'm seeing across different machines on my LAN, here is what I get:
[G:\util\misc]python speedtest-cli --server 17394
Retrieving speedtest.net configuration...
Testing from Bell Canada (65.92.109.81)...
Retrieving speedtest.net server list...
Retrieving information for the selected server...
Hosted by Bell Canada (Toronto, ON) [331.11 km]: 17.667 ms
Testing download speed..........................................................
Download: 301.52 Mbit/s
Testing upload speed............................................................
Upload: 117.86 Mbit/s
The d/l values match up pretty well to what I'm seeing on other devices, the u/l however is significantly smaller. I will look into the window sizing again and see what Lars' comment might uncover.
Thanks again, this is definitely the better (more stable for sure) way of testing.
EDIT
====
Found the following testing site (
http://www.dospeedtest.com/) that matches the Python script CLI test results pretty closely, so at least on my machine with FF 45.9 executing the test it appears to cause no adverse effects.
Also, I put together a quick speedtest.cmd file (see attached) that calls the Python script Dave provided and tosses-up a few entries you can chose from (runs a default test against a BELL server, which is one of the big national tel providers in Canada). You can pick your own servers of course, see the results of:
python speedtest-cli --list
...but that will be a long list, so you may want to pipe this into a file or a page-at-a-time view:
python speedtest-cli --list |less