The bug is best seen with long multi-level demos (like TNTmax.lmp ;) or very long single-level demos (like TimeOfDeath's Nostril.wad UV Maxes),
Using -skipsec, -warp, the End Level key or the Next Level key makes the progress bar (and the demo) advance pretty slowly. However, if you ALT+TAB out of the program (or use the WIN key), the demo skips forward MUCH faster.
Hold the ALT key and press TAB once while the demo is skipping and see the progress bar fly. Shouldn't demo skipping always be this fast, without having to use ALT+TAB?