I see your point.

Even though from my perspective Qt is supposed to be heavily widget based.
Otherwise I would use SDL or embedded Opengl.

I imagine your approach is faster.
But, if I want to be fast I'm using opengl and I'll achieve 500 fps.

My goal is : creating a scalable and easy to maintain desktop application.
From my point of view using widgets and only widgets makes more sense than delegating the painting. This should be done only to customize the appearance.

And beside, what about layouts in a delegate ?

I can't help but thinking it's a hack to make a painting area "look like" a widget and I don't like the idea. And that's not the direction I'd like Qt to evolve.

Because it will be many times faster. 100 items means 100 buttons in your case, each of them needs to be repositioned, shown or hidden as necessary by the window system. Using itemview approach and only drawing the button (without ever creating the actual button) makes things much faster and much easier.
Why would rendering a given scene vary between a delegate and a widget ?
It should be as fast in a widget than in a delegate period.

If I'm displaying a button through a qwidget item or through a delegate, the only thing that should vary is the memory occupied by each instances not the rendering speed.

I guess that's what they're trying to achieve for the next Qt.