Speed of scrolling with a background pixmap

Has anyone noticed how slow scrolling becomes when a background pixmap is used? Let me define what I mean: scrolling speed == press pagedown in a large buffer and let auto-repeat do its work (fast auto-repeat assumed). speed is the number of displayed pages per time unit. background pixmap == a background-pixmap used on the `default' face, e.g. with set-face-background-pixmap, or with XEmacs.default.attributeBackgroundPixmap X resource. nothing fancy (e.g. having different faces have different bg pixmaps is fancy). Whether you use a bg pixmap or not, try comparing scrolling speed with and without a bg pixmap. For me, scrolling is much slower without a bg pixmap, even though the pixmap itself doesn't scroll. Can you repeat that? Any ideas about the reason?