I think, while there still is space for improvements in the C2 engine and also in the runtime enviroment (Chrome, NW), many problems are (from example capx i saw from others) user-made or because the lack of hardware support.
The biggest problems from user side are:
- overuse and misuse of Physics (really, many things you can easy code in 2 or 3 lines, but some developers use physics for everything / also mixing incompatible moving (like 8-direction) with physics) Advice: only use physics if there is really no way you can get the behavior you want in code.
- overuse of WebGl effects (effects are very heavy, try avoid them as much as possible, most things can be baked in the sprites/background images already)
- overuse of Particles (not much to say here, but keep in mind: a bad game doesn't get better because you use explosions with thousands of particles...)
- using way too big image resolutions (i remember this now, when reading another post) Always use images in the smallest size possible, specially for mobile games, because many small devices still have memory & gpu power problems with over-sized images. You really need a 2048x2048 background picture for your 2d retro platform game? I think not...
- logic errors/coding errors (like for example putting stuff in the system.every tick that doesn't belong there)
I don't say that everybody here makes this mistakes and that there aren't problems with the construct engine itself, but from what i experienced around 80 % to 90% of the users that have problems with their games, have them because of misusing the engine like i show above. The biggest selling point of C2, it's easy way to use and make games, is also the biggest problem of it, because many people without knowledge of the underlying technology or even coding are attracted and if something doesn't work like they want, for sure they blame the engine first.
There is another issue from the hardware side and this is lack of WebGl support from drivers (video cards)
Your shit little 80's arcarde game doesn't run well on your big pc or on your mobile, if your gpu doesn't support WebGl at all or good enough, and there is not much Scirra can do about. Because of this we have this big difference between all the devices we are testing our game. But because all GPUs made in present time support (hopefully) WebGl, this issue will vanish.