The Windows vs Haiku case you describe is mainly a difference in terms of development culture. In Windows you can get similar fast performance and responsivenes

The Windows vs Haiku case you describe is mainly a difference in terms of development culture. In Windows you can get similar fast performance and responsiveness by using the native GUI functionality, but most desktop applications nowadays use toolkits like Qt (at best) or Electron (at worst) that reimplement a ton of functionality that the OS itself already provides and treat it as something different than what it really is. On the other hand in Haiku everything uses the native Haiku APIs. But from what i've seen ever since Qt and Java were ported to Haiku, a bunch of applications started relying on those to run on it instead of using the native APIs (which IMO defeats the entire point of Haiku). Haiku does use a lot of prewritten libraries BTW, they're just hidden behind the scenes and shared among applications instead of each one bringing their own copy.