saiyadjin has a very good point. Yes, there are performance issues with browser engines that are not our fault...but I think there are a lot of problems that simply stem from bad design, and a lack of understanding about what limitations mobile presents.
I think a lot of that bad design stems from people thinking that, with C2, you can be lax about how you design your game, because the editor and the engine will take care of you.
A lot of people that they can build a full game as easily as a prototype.
It's a lot more work. A LOT more work.
You have to plan things out, organize your events sensibly, take notes, keep a changelog, modularize your logic to avoid the nightmare of WET design, make sane intelligible comments, keep those comments up to date (harder than it sounds), work out a consistent workflow for asset management, understand best practices for triaging bugs, understand common logic patterns, ...the list goes on...and on...and on.
The difference: with 'proper' game engines, you have to know most of this stuff to get anywhere in the first place. C2 is more lenient, but because of that people end up with inefficient, redundant, brittle logic that is poorly organized and nigh indecipherable. This still works...until something breaks and they have to try and make sense of their hot mess.
At this point the whole project generally falls apart, and there is rarely any way to put it back together short of starting from scratch...hopefully with a clearer vision of the goal, and a better understanding of how to put the pieces together.