when I test Wendel's example and set it to False I don't get the lag. When I set it to True I do.
In one of the earlier threads on this issue, it was suspected that the problem might be connected to ATI drivers. So it could be that the DX drivers for your nVidia card and those for my ATI one have different behaviors. If so, perhaps yours work fine without the "magic" and are harmed when it's used, while mine need it for decent performance. Weirdness for sure!
Maybe for now we could offer this workaround to users/players (in menus/options, or whatever) as a "recommended ON for ATI, recommended OFF for nVidia, but see which works better for you" deal when we write our BlitzMax apps/games? Not ideal, but better than nothing if no built-in BlitzMax fix appears or is possible (since the problem seems(?) to lie in DirectX/drivers rather than BMax)....