Being a good developer is indeed the easiest way to avoid performance LOSS.
But if you're talking performance GAIN, then I don't see any way to make JavaScript outperform even C#/.NET without directly editing the garbage collection of C2 to match your specific game and compiling the whole thing from C++ into WebASM/Emscripten.
Now comparing C2 against itself we already know there's room to GAIN as well, because after years of being "fast as native" Construct 3's runtime is about to be even fast-as-native-er, which is actually awesome and I hope that goes well for Scirra and pays off with more subs for them, because it is sorely needed, especially in the areas where the improvement are occurring.
But that also kinda disproves the whole "I can make your game 0% of the CPU!", because unless you're making a turn based game that does nothing until a mouse click is made it's simply a catchy phrase without fact.