@Ian: In fact, I get the same or very similar drawing duration of the single-window

in my test, which I qualified "a few tenths of a second". It's very perceivable, there's no doubt.

Antialiased oblique lines are also very perceivably better than non antialiased ones.

My point is that the doublewindow test is a very extreme situation by drawing a very large

number of antialised lines.