why not Dosbox with overlay and composition

If i remember correctly DOSBox uses SDL, another possible slowdown ..

@Hans

I read it as we talked about 3D support in DosBOX, not as a way to display a screen, that's way I wrote what I did, hope this clear the confusion.

There are lots of 3D games you might run in DosBOX if supported 3D, but composition does not provide sharers nor Z-Buffer or any of the stuff you need, MiniGL/Warp3D/Gallium3D is better for 3D stuff.

Yes as I wrote previews comment where I explained how it worked in Basilisk II, I also explain its excellent for scaling.