It is faster to draw one point than to draw 1,000,000 points even in directx.
You MUST agree or you dont understand the concept.
It is faster when you draw on a Screen DC/Graphics
Direct2D Render Target objects are Double-Buffered natively
and all drawing commands are done on an offscreen surface, which is presented only once
So, 1 point or 1 Billion of points will take the same time to be drawn (on ID2D1RenderTarget::EndDraw)
Like Double-Buffering in GDI with BitBlt (Memory DC) or GDI+, but a lot faster, because managed by the GPU