Correct way to monitor network speed (upload/download)?

Hello, folks!For a few months I've been using speedtest-cli to monitor my client's internet speed (upload and download) and everything was going just fine, no issues, until last week another client got in touch with me informing the data that was being shown for them was not correct since they did use another online tool to monitor their network speed and it gave higher values than the ones we were getting from the script in the server. I ran several tests in several different servers and they were all giving me speeds in the range of 3~4 MB whereas online they would get 20MB up and 40MB down.Due to that I took down the service from such client and started to try to find another alternative for such monitoring. Do you guys also monitor your network speed? Talking about the speed your ISP gives, not just incoming/outgoing traffic.

Checking speedtest's GitHub there's this message at the end of the page:

Inconsistency

It is not a goal of this application to be a reliable latency reporting tool.

Latency reported by this tool should not be relied on as a value indicative of ICMP style latency. It is a relative value used for determining the lowest latency server for performing the actual speed test against.

There is the potential for this tool to report results inconsistent with Speedtest.net. There are several concepts to be aware of that factor into the potential inconsistency:

Speedtest.net has migrated to using pure socket tests instead of HTTP based tests

This application is written in Python

Different versions of Python will execute certain parts of the code faster than others

CPU and Memory capacity and speed will play a large part in inconsistency between Speedtest.net and even other machines on the same network

Issues relating to inconsistencies will be closed as wontfix and without additional reason or context.

Edit: we currently use the server which has the best results for such link, we run around 10 to 15 tests for each link (which are generally two or three WAN links) by changing the default route on the firewall (SonicWALL) since just getting the one with the best latency may, or may not, be the best option.

Edit 2: checking if the problem is not related to their network interface card being limited to a specific bandwith speed. Someone at work suggested me that and maybe that could be the issue.