I'm always happy to investigate the performance of any .capx files if you think the C2 engine is the bottleneck. Usually however either people complain but don't send me anything so there's nothing I can do, or they send something and it is clearly bottlenecked on the hardware, e.g. hammering the GPU fillrate or ramming it full of WebGL shaders. In that case a native engine won't help since you'll just run in to the same hardware limitations. This is part of the reason I am very skeptical of claims to move to a native engine to solve performance issues - none of those hardware-bottlenecked games will get faster.
Note that if it occasionally drops frames, that's not the same as a performance issues, that's more to do with the browser v-sync scheduling. Chrome has had some issues with that lately, but they're getting a lot better (Canary is already much improved for me). In these cases the engine/hardware are perfectly capable of hitting 60FPS, or possibly even much higher, but timing errors in the v-sync scheduler mean it occasionally fails to schedule a new frame even when it has the resources to do so. That should not be confused with performance issues where the engine or hardware do not have the necessary resources to reach 60 FPS.
Note that single frame Sprites with no behaviors are specifically optimised to be zero overhead in the C2 engine.