but its the original version? iirc there is a mirror that would then be calculated in software - hence more load..

yeah its the original dos version. So is that just normal behavior..? and yes when i get the the mirror the fps really drop

the mirror shouldn't be the problem as Build renders it like an extra room with extra sprites and there's similar "portals" all over the game that provide connectivity within levels. If there's anything that should give Duke3D a performance dip on that hardware, it's any translucency effect in a high resolution mode at a close up, as that may require expensive reads from video memory to write blended pixels.

Also i'm not certain if a Geforce6600 has any VESA 2.0 VBE extensions because that could also play a part for performance, and since that's way out of the timeline of Scitech's software, UniVBE/SDD can't help you there.

I really hope i dont have hardware issues

I doubt it. Your computer probably just long since abandoned the standards that allow Duke3D to run quickly.

Use Fastvid under MS-DOS which will speed up your video performance. Fastvid makes the computer in my signature run Duke3d much better.