do I just hear the arguments again how java is slow vs c++?

the browser is the jvm of a web app, very secure and js engines like V8 is extremely capable... although js programs can be run at near native speed (the browser is a c++ program normally and there is native compilation of some code fragments)

you should also make yourself familiar with webassembly tech... some folks use even node.js on the server side... the js UI model is also very good

and nobody is replacing native apps, there will be a great competition and in those areas where native has competitive edge, it will win