It's up to the site admin to configure it that way, but it's possible some IP ranges/user agents are more often used by bots and therefore have an increased weight.

For old browsers there's also an option to use meta refresh instead of JS (https://anubis.techaro.lol/docs/admin/configuration/challeng...) but that's quite a recent addition and not enabled by default.

I'm currently roaming in Finland with a Spanish SIM so would have expected the opposite in that case.

This page loaded pretty much instantly (certainly in the time it took to switch to the background tab I loaded in). But then ffmpeg is written by old school engineers with old school ways of working. Their social media accounts are a hilarity of trolling worthy of slashdot in its peak.