You can write all of your Qt logic in javascript if you want to. It will probalbly still be much faster than a browser. But the thing is that Qt allows you to write your logic (and your ui) in C++ if you need to. It gives you somewhere to go when performance is an issue. And on mobile or embedded there is a high chance that only a compiled, no-vm, no-gc language like C/C++ or Objective C or Rust will give you the performance you need. One thing is for sure: If you have to build a cross platform native application be sure to first prototype the performance and processor/battery use of your technology choice. Even if you don't choose something like Qt at least you can write your backend in C++ and write a different native front end for each OS. But if you use e.g. web technology and don't prototype first there will be absolutely fire escape later. A lot of people have learned this the hard way.