Thanks Rishabh. That’s the next step I’d take too. Comparing the data the test pushes per second with what the tunnel can actually carry should separate a bandwidth limit from an MTU problem. [If you’ve done this before, I’d like to hear what numbers you usually see.]

I don't use one fixed number, since it depends on the VPS and VPN type. I compare iperf3 throughput with the test's required throughput: if iperf3 is lower, it looks like bandwidth, and if iperf3 is high but large packets stall, I lower MTU toward 1400 and retest.