C3 runs in the browser, with all the benefits that brings, but also with it comes all the negatives. i.e Construct 3 won't be as fast in performance as Fusion 2.5-3 , GameMakerStudio 2, GameSalad, Stencyl, Unity.
That's not actually exactly right: in some cases the browser tech is very advanced and can exceed straightforward native code. For example Chrome has a sophisticated parallel rendering architecture, so all WebGL calls are forwarded to another thread and run in parallel while the next frame starts rendering. This essentially lifts the performance overhead of the graphics driver off to a different core, which can actually result in improved performance vs. a native engine that does not implement such parallel rendering. It's a difficult and complex feature to get right, and we certainly didn't try for the CC or C2 editors. So this aspect is probably faster because it's in a browser. On top of that we moved from OpenGL 1.1 in C2 to OpenGL ES 3 (WebGL 2) in C3, and C2 has a lot of JS <-> native transitions which is expensive performance wise (and can more than undo any performance gains in the native code), and C3 has no transition overhead because it's all in JS.
So you can't just say "it's slower because it's in a browser". There's a lot of tradeoffs going on, and in some cases leveraging the browser tech brings improvements even over native code.