DBPro is slow - GSDK isn't, because it's just the 3D engine, and none of the other stuff (code for while/for/repeat loops etc), that just isn't needed. When the GSDK first came out, I worked out that there was a approximately a 175% speed increase between DBPro code and C code with the GSDK...
I don't mean code execution speed - of course C/C++ is faster than DBPro in that respect. I meant that DBPro's actual ability to render complex scenes, even with the most basic render loop, was very slow.
Now Darkbasic is supported by MS.
It's not supported by MS, it's promoted by MS (basically as a freebie addon to Visual C++, though it's not free for commercial products as you know).