Hello,
1 - One user reported similar problem with Safari on his Mac (View in Safari Browser). I suspect these were caused by lack of hardware acceleration for WebGL content.
Could you check if you have the same performance problems in different browser and if other WebGL sites work well on your machine (for example: three.js examples)?
2 - iOS doesn’t support the full screen API (Can I use... Support tables for HTML5, CSS3, etc) The only workaround is to add a home screen icon from Safari with a link to the scene (open the scene directly, for example: 3D scene , tap Share, Add to Home Screen).