Plz. explain about the style's in Qt [QSS, QStyle]

[nen777w]

I have the Qt widget-based application, and I want to create a modern UI.
In the internet I found the file with QSS styles, that I want to use as base.

But bumped with the following problem:
My application has too much widgets, and when I run it with the style
@
app.exe -stylesheet=darkorange.stylesheet
@

it runs a very very slow.
If I understand correctly, it happens because the Qt compile the QSS to each created widget separably.

Is it possible to fix or speedup it?
(for example, with the my style class that will be derived from QStyle)

Thanks.