i strongly believe it has to do with wrapping the entire UI into something called ETO to unify both mac and windows.
the result is laggy tool panels when dragging, specifically when you click and drag there is always a slight delay. it has been complained about really many times and nobody ever acknowledged it as far as i could see. it might also never have been communicated correctly.
i made a short video that shows what bugs me personally the most here. below the first video is in v8 which exhibits 2 issues which might belong to each other but important to notice that there are 2 issues:
1st issue is when you move the tab out of the container it now lags even harder than before, you can see how the container/panel lags extremely slow behind the mouse which moves at a normal pace.
2nd issue is later in that video you see when you look carefully if you drag the entire panel there is always a slight jump it always needs a split second to pick up the movement, it is pretty annoying and might not even come across on the screen so well but if you look closely you might even pick it up here.
as a reference v7, dragging the panel works smooth.