Seems like you're playing in Fullscreen (Hardware) mode! Try changing this setting in the options, choose Fullscreen (Software) instead. As far as I can remember, and this is back when I used to play this game in 2002, hardware mode has always been glitchy. It just smoothes out textures (weapons, effects...). Software is more pixellated and you can see textures seams and artefacts, but it's not like the game's a beauty to begin with...

However, for hardware mode, there's a partial fix from dgVoodoo's author (a neat wrapper which can help a lot with graphic issues), here is the info if you want to try it (I haven't yet):
https://steamcommunity.com/app/32630/discussions/0/135508398031790648/ (short guide)
https://www.vogons.org/viewtopic.php?f=59&t=52136 (details, the last post is probably right though, HW mode for DF2 isn't really an option)

Edit:
Just tried the fix - well, it's still glitchy, way less than vanilla though, but still distracting when you least expect it.
As for Turbo mode, I believe it's meant to accelerate the game on old configs by making terrain less detailed, not to boost the image quality. So off = best details.
You can still get a bit more image quality in Soft. mode anyway by opening df2.cfg (in the game folder) with a text editor, and under //DISPLAY, game_res = , write 6 (1024x768) instead of 5 (800x600). 7 (1280x960) crashes for me even with dgVoodoo set up.