Build engine is just very CPU intensive to emulate. Duke3D has lower requirements, and I'm having to play it at 320x200 on a notebook with 1.3GHz Core 2 Duo. 640x480 results in 20fps gameplay. Though DoxBox 0.74 might improve things over the default 0.72.

Yes I know about EDuke 32, I just wanted to know how well the game would perfrom here.

OK, this thread got me working, although I still might try some tweaking. Using machine=vesa_nolfb did the trick, at least for me. Thanx!

I solved my fluidity issue just totally uninstalling the game and then reinstalling it again (with its standard-gog video configuration).
It seems it starts to lag whenever I modify graphic settings.