Generally platforms providers optimise graphics highly for their own platform. A generic cross platform library rarely invests similar effort for every platform it can run on. The usually applies more to mobile however - most desktops require even the platform providers implementation to be somewhat generic.

The other part of the inefficiency is more to do with what the cross platform engine is internally guaranteeing vs what the platform providers will guarantee. For example, let’s say skia here always runs at 60fps. Great, and a lovely user experience - but does it then respect low power mode in iOS or Android, or does it just continue at 60fps eating away through battery power because it’s geared to the common case of all platforms?

I’d love this to work, but years of the reality of using cross platform renders has made me quite skeptical of their ultimate benefit to the user. They’re great for developers though - but that’s the wrong way round for convenience usually, I think.