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.