Pickman: I tested it again with direct3d rendering (via the GOG-graphic-setup tool) and i think this works better than the openGL i had in my previous session.
Don't get me wrong, the game is absolutely playable, I am just wondering why a game this old could not be made to run faster and with constant framerate. I messured it with FRAPS. Maximum fps are around 25-30, minimum 10 frames on open water with mana balls and reflections ON, 20 with reflections off.
On the other hand, i played it JUST LIKE THIS when it came out years ago, so at least the feeling is 100% accurate:)
Could you try with Overlay and Surface too? Also, are you using the hi-res or lo-res mode?
I remember playing this game on the old Pentium 100mhz, but it didn't feel like it slowed down too much. Well, maybe when using meteors. Otherwise, it was usually fine.