Hi! I am also having this same issue on my sites where the high quality video I upload looks fine on desktop but low quality on mobile.
Gerda, I tried another video insert and that kept the quality but didn't allow me to expand the video to full bleed, so it just looked tiny. When the video expanded in the player, it looked great. But the tiny video isn't what we wanted.
Squarespace devs, any way to fix? Current site traffic is 80% mobile.
Many thanks!
AuraKat