First, you've to determine what exactly your performance bottleneck is. It would be helpful if you can give us your system's specs, most importantly the cpu and gpu specs. The bottleneck may either be in your system, your code or jPCT. In case of the latter, there are some things to consider before reducing polygon count (which will always help). You are rendering a landscape IIRC? If so, are you using octrees for that? How is your landscape organized? How many polygons do your models have?