because you should really look into these frameworks if you really think its a good comparsion. if you exclude all the unserious java frameworks you will be stuck with vertx, after that there is a huge list of c# and than there is the rest of java. I mean yes, these frameworks might be the next big thing in the java world, but once they are there, they will at least loose 20% of performance. (p.s. you can also remove the first c# entry, because no sane person will write everything as a middleware)
If vertex's json serialization is too slow, you swap that part for something faster, that's how you use languages in the real world; what is this language capable of, and how i can fully exploit it
Developers nowadays seem stuck in a box for some reason