> I find fast.com massively overestimates my speeds.
I find fast.com more accurate than most, going by the throughput I see to/from other resources, particularly for upstream rates (which some, including Cloudflair, seem to significantly under-measure). Of course this may vary depending on line type (FTTC, “up to 67Mbit down n& 17 up”, generally seeing more like 50/12) and location (due to differences in local network conditions and peering between you & the speedtest) to I'm not surprised to hear you see the results quite different to those I experience.
> if you hide the tab running their speedtest you get even higher speeds
I suspect this is due to your browser throttling CPU use and timer granularity for background or occluded tabs⁰ reducing the accuracy of the readings, if the code isn't working to mitigate the effect of these throttles on the calculations.
I find fast.com more accurate than most, going by the throughput I see to/from other resources, particularly for upstream rates (which some, including Cloudflair, seem to significantly under-measure). Of course this may vary depending on line type (FTTC, “up to 67Mbit down n& 17 up”, generally seeing more like 50/12) and location (due to differences in local network conditions and peering between you & the speedtest) to I'm not surprised to hear you see the results quite different to those I experience.
> if you hide the tab running their speedtest you get even higher speeds
I suspect this is due to your browser throttling CPU use and timer granularity for background or occluded tabs⁰ reducing the accuracy of the readings, if the code isn't working to mitigate the effect of these throttles on the calculations.
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[0] refs: https://blog.chromium.org/2020/11/tab-throttling-and-more-pe..., https://www.reddit.com/r/incremental_games/comments/jefviz/f..., and many more