Maximized windows extend slightly beyond the defined desktop area - that's just the way Windows does it. You can reduce the overlap by extending your desktop borders slightly (jdesk, desktop2) so that there is more space between the edge of the theme's elements (bar, etc.) and the windows. Non-maximized windows will then have a small gap there, maximized windows will fill the area completely. The overlap lag is probably due to a transparent or alpha-shaded component in the overlap region. That'll put more stress on your CPU and/or graphics card.