Malice I think problem is not related to the scaled big wall with alpha texture, maybe something related to those 'far away' objects? Are they out of the clipping range? Cause I don't really see any reason for a simple model with alpha texture to 'kill' framerate. I'm using a lot of models/sprites with alpha texture and never faced such problems.

Why don't you use sprite instead? It will looks like the model but will render faster I guess.

Greets