Ok, as said yesterday I've expermimented a bit and here's some info that might be of interest:
I tried another JJ2 engine game, Animaniacs A Gigantic Adventure, and the same issue happened there. Windowed works fine, Fullscreen is slow, fullscreen with hardware-accel is sloooooowww.
Next I started the GBA emulator Visual Boy Advance, since it has many rendering settings. And choose to use DirectDraw. And yes, same FPS drop occured! But in full suprise, disabling
Triple Buffering for DirectDraw made it run at 100% speed! I'm not very technical on this, I have no idea how triple buffering could interfere with frame processing in such a matter that it drops more than half.
A Google search didn't gave me anything usefull last week, but now it gave me a link to
this thead on some Windows 8 discussion board, which describes the exact problem. Unfortunately, the advice in that thread didn't fix JJ2, yet it sure pointed at the right direction.
Just like Windows Vista/7, Win8 comes with an integrated version of DirectX (11.1?) and is missing some libraries from good old 9.0c. Some games rely on those libraries, and installing a DirectX package using the web installer or a standalone package always did the trick. With Windows 8 however, it's the first time the web installer detects a higher version on the system and aborts the installation of the 9.0c package. So we're forced to manually download and install the stand-alone package now. Thank you, Microsoft!
So to get back to the point, does JJ2 use triple buffering in fullscreen? And can I disable it somehow?