Comment 5 for bug 1813354

I don't remember for sure which article it was, but I was travelling at the time and looking to auto-select the fastest mirrors. Searching now finds a familiar-looking article and forum answer:

https://linuxconfig.org/how-to-select-the-fastest-apt-mirror-on-ubuntu-linux
https://askubuntu.com/questions/39922/how-do-you-select-the-fastest-mirror-from-the-command-line

This person seems to have issues with using the 'mirros' solution as well:
https://askubuntu.com/questions/319433/making-mirror-mirrors-ubuntu-com-highly-available