It would be absolutely dastardly -- but could actually have the effect of forcing app developers to care about performance :) They won't care unless they have to. Web developers have to care about performance because of SEO (and click metrics...) but it only goes so far because it's not forced.
I disagree completely with the walled garden model of iOS (and the direction of macOS) but, if it nonetheless exists, I'd like to see them do something hardcore like this. The performance and battery gains would be out of this world.