So I had some more time to work on this. I tried a new 4.0 build with OpenGL. But I still couldn't load custom textures without lag. Could it be I'm using the wrong options? I selected the ZTP hack... what else is there?
After that I tried resizing the HD texture pack. I used GIMP with the BIMP plugin to resize all textures to 50%x50%. But...... then only the sky would load in the game, nothing else! Apparently it doesn't work like that. Can anyone explain to me how it does work?
Thanks.
I'm about to abandon the whole HD TP idea in favor of playing it on the GameCube... 
After that I tried resizing the HD texture pack. I used GIMP with the BIMP plugin to resize all textures to 50%x50%. But...... then only the sky would load in the game, nothing else! Apparently it doesn't work like that. Can anyone explain to me how it does work?
Thanks.
I'm about to abandon the whole HD TP idea in favor of playing it on the GameCube...