There’s no glide support in Magic Dosbox yet, so I believe it’s a placebo effect. Dosbox Turbo has rudimentary 3dfx support, but it’s so incredibly slow, you’re better off using SVGA. Let alone the crappy sound emulation and other bugs in DTurbo.
The problem is that there is no Glide wrapper for Android yet, so the whole 3dfx/Glide code has to be sofware rendered, and is not executed natively on the gpu. That’s why it’s completely useless atm.
I would so love to play Tomb Raider or Quake in 3dfx, and I had lenghty discussions on the matter with Tony (Magic Dosbx dev). But there simply is no way for proper 3dfx emulation yet, unless someone comes up with a Glide wrapper. 🙁