Speed question

Using iperf3, I can see that my LAN speed from a host to my (virtual) openwrt router are in the range of anywhere from 600-850 mbps. From that router, which is connected directly to my (brand new modem), using speedtest, I am getting in the neighborhood of 250-300 mbps most of the time. The modem is a Arris SB8200, so these are good speeds (good enough for me, anyway). I am satisfied with the performance of both inter-LAN speeds and my router's Internet speeds.

However, when I run the same speedtest from a different host on the same LAN, I am seeing speeds of well under 100 mbps, typically around ~50 mbps, sometimes a bit higher.

I realize that running openwrt in virtualization may have some issues, but I am able to repeat these tests and get consistent results. If the problem is with virtualization, then I would think I could expect some speed degradation either on the Internet or LAN side, but I don't. It seems more like there is some issue either with speedtest, or some problem with my router configuration, or maybe openwrt itself (much less likely, I agree, but still a possibility from my testing's POV), or even perhaps some other issue.

For the most part, I do not notice much latency or issues with the overall configuration; I am able to watch Youtube videos (carefully rewinding and re-watching sections that seem to stutter or pause, and I am able to confirm it is the video, not the TV or my desktop). I am able to sync files to the cloud and surf the web with little latency (though I do note that firefox seems to be getting clunkier and clunkier these days, with initial page loads taking some time).

Again, I am satisfied with the performance of my network, but I am wondering if there is an explanation for why there is not consistent internet speed across the LAN. I am willing to try all constructive suggestions, short of replacing hardware; sorry I am already over budget for several months and need to slow down my spending for a bit.

I have tried increasing the vCPUs assigned to the router in hopes that would help. But so far it hasn't, and folks in the IRC have told me that is unlikely to have much effect. (But I'll try again if you insist.)

I am running openwrt 21.02.2, btw. That is a very recent update, and I noticed this same issue with 21.02.1. (I have not done thorough testing on earlier versions though.)

May have something to do with how the network adapter on the device is optimizing ethernet connections.

Using different versions of iperf3 on the sender and the listener can also produce widely different results.

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There is more than one adapter--one for upstream, another for downstream (and actually, another for admin, but that is not involved in this). But even if the adapter(s) were to influence optimizations, I'd expect that would be true for all packets crossing the interface(s)/adapter(s).

I am using the same version of iperf on sender and listener (2.0.13), which I think is the latest. Thanks for pointing this out, as that could definitely lead to issues.

Just as a note, I am also using the same version of speedtest (2.1.3) on all hosts involved in testing. I had to copy these scripts (python) to the routers from a host that has the latest version; the 2.1.2 is broken. I guess it is possible that a change in version of speedtest could have some kind of impact, though I think that would be more true for binaries. In this case, the speedtest scripts are python.