So ive seen construct's 2 preformance vs stencyl and consruct was like 5 times faster. Is this for html5 and flash as a whole or it is just these engines preformance.
Develop games in your browser. Powerful, performant & highly capable.
You have to be careful to not compare apples to oranges.. general statements about performance are hard to make, it depends a lot on the specific use case, the actual content and whether or not that content is optimized for the device it's running on (with all that this implies).
Html5 first advantage over flash is that it can run on moderns browsers without any plugin (that also is true for mobile browsers, since flash for android and maybe iOS (not sure for iOS) has been discontinued as far as I know, performances wise it would be better to compare PCs only), as far as I saw myself, as long as webGL is enabled the game can run nicely on computers, except for really weird specific cases (I remember some people having beefy computers that had issues running C2 games with WebGL while my bottomline laptop could make them run with no issues, weird), meanwhile flash games I cannot say for sure as I am not a big user of flash games (the demo of binding of isaac was pretty slow though sometimes, but I cannot say for sure if it is a flash issue or just bad implementation).
Hope that helps a little.
Also another feature/issue of html5 : browsers supports it, and sometimes they can f*ck it up for quite some time as history showed us, also some features may be first not supported across browsers (rare in the case of C2, but I saw it happen).