I don't get a slowdown with Jabo's D3D8 1.6. If you video card supports it, it should be fullspeed. Disabling 'Self rendered textures' in Jabo's D3D6 plugin will fix it too but it'll break other effects in the game.
Other plugins that shouldn't slow down here are Rice's Video, glN64(), Direct64 and maybe others, so try these as they may work better for you.