1: In my experience the skills matter in some cases. I once made a game, which had a game world of about 20’000px with one big masked tilemap for the ground. It worked with about 20fps on a Nexus5. One I split them programmatically into pieces of max 1000x1000px and enabled rendering dynamically for each visible/hidden piece, it run with 60fps.
But smaller and simpler games should work just well without optimization.
2/3: I think this link helps you.
I believe Phaser 3 is the easiest and best way to make 2d games for mobile.