I saw this myself when this was news. Created empty firefox profile, installed only userscript changer plugin.

Default user agent - rotating loading circle before video starts playing. Windows/Chrome user agent - video starts immediately.

Tried with multiple videos, changing first user agent that opened the video to make sure it’s not cached somewhere.

Didn’t bother to install Chrome for reverse test though.

Now it’s back to loading at the same speed regardless of user agent though.