CJS is a wrapper. It's actually a Canvas with WebGL engine, or rather, half of what chromium is. On iOS8+, it's now much slower than the Safari browser engine and around 5-10x slower than the new WKWebView Safari engine.
On Android, its about the same as the more recent Crosswalk version for my big games. The problem with Crosswalk is it can be stuttery or jitter on some mobile hardware, while running smoothly on others. It's quite strange. But then with my experience with CJS, on Android, it just crashes on a whole range of devices, including Samsung Tab2/3, S3 phones..
CJS with WKWebView is unmatched for iOS though.