I guess I'll start by saying that JS is my absolute favorite among all the top languages out there (StandardML, Rust, Scheme, and CL probably being the others in my top 5). Thanks for making it!
Of course, everything said is hypothetical as we can't actually wind back the clock.
You could have rolled your own Scheme instead of adopting a then-fast implementation. The Scheme would certainly have been faster from the very start if for no other reasons than integers are faster than floats and real arrays are faster than hashtables.
Scheme would likely have been a bit faster from the start, but the real question would be the trajectory from there. JS needs inline caches and hidden classes to be fast. I don't see how all that could have possibly run on typical systems of the 90s. In contrast, there's a lot of Scheme optimizations that would work perfectly well on those machines before resorting to the really heavyweight stuff.
Considering that JS with many millions in investment is generally slower in typical code than something like Gambit or Chez (both of which have very little financial backing) leaves me assuming that the Scheme trajectory would always be better.
This just leaves the hypotheticals of Scheme's other effects.
Anyone who's used basically any lisp for web work would understand why HTML would become effectively subsumed. S-expressions make dynamic manipulation of the raw trees very easy as that's what a lisp is designed to do. React revolutionized frontend development in 2013, but a huge part of what people like most about it would have been obvious to web developers years earlier.
XML may well have never happened either. While JSON was apparently only obvious in hindsight, the idea of sending S-exp over the wire then parsing out the data is very in-your-face with lisps.
CSS is a much more simple matter. One of the major syntax proposals used S-expressions and their easy integration into Scheme would have all but guaranteed their adoption.
WASM likely wouldn't be needed because as has been shown with Common Lisp, you can already get very low level with just lisp (and even lower if you allow stuff to optionally handle its own memory). Asm.js got really complicated to the point that WASM made more sense, but with performance already being very good, it becomes a lot less necessary (even without those lower-level changes).
Flash was a bit hyperbolic. It would probably still have existed for a variety of reasons, but probably wouldn't have become a separate language and ironically would probably still exist today as a development layer over the browser's features.
Steve Jobs was very open that he didn't want an app store, but instead wanted web applications. There was an issue with the browser's capabilities, but the biggest issue was that JS engines were still just too slow in the late 2000s (part of what killed off the very amazing webOS). Scheme would likely have been quite fast by that point making the web app argument much more appealing and possibly saving the world from fractured ecosystems.
Of course, there's the counter-argument that the world would never accept a scheme and it would have either been replaced or moved everyone into the nightmare of Java applets. There are certainly a lot of alternative universes where the world would be far worse if we'd wound up with Scheme rather than Javascript.