Hmmm, I'm still doubtful. I stumbled across this while trying to find performance information on Ejecta. Note: I'm not using Construct 2 or any Scirra products. I ended up manually creating my own test.
We created a WKWebView with an HTML page, an optimized RequireJS bundle, threejs and a couple other resources. We were getting 18 fps in a scene with 2000 threejs plane geometries and 9 materials with textures.
We create an identical Ejecta scene but with an index.js instead of an html page, and we got 50fps. Also note we are merging geometries and have interactions with them, etc.
JIT compiling of JS is great and would likely make Ejecta faster; however, I don't know that JIT compilation is the only performance metric. It is likely that the browser has lots of overhead and memory consumption. It is also likely that WebGL is slower than OpenGL ES on mobile devices. This is also exemplified by just running any threejs demo in your browser (try buffergeometry, which should the highest performing, and runs at 7fps on an iOS8 ipod touch).
It could be the case we are doing something wrong. I'll retest eventually and get back to you.
Devices tested: iPod Touch, iPad Mini, iPad Mini with Retina