This seems to use speedtest-cli which is notorious at reporting bad values.
Using speedtest rather than speedtest-cli I get a download speed of around 332Mbps, which matches my broadband link. Speedtest-cli, and hence this container, reports a download speed of 74Mbps.
To double check, I run speedtest from within the container and it still reported 300+ Mbps compared to the 74Mbps of speedtest-cli.