When a user is having any connectivity issue, have them go to https://www.fast.com (this is powered by Netflix).

You might have them test without their VPN connected (and maybe also with their VPN connected).

The results should (if they click "show more information") show:
  • Speed (download) - Mbps
  • Speed (upload) - Mbps
  • Latency (Upload) - ms
  • Latency (Loaded) -  ms

Example:

These bits of information may shed some light on what type of service they have on their end.