Few things you can have a look at:
First off make sure you are running java 64-bit, it really helps for minecraft.
Secondly, try and give java more memory to run when it runs pixelmon, you can modify java arguments in the minecraft launcher (which I'm assuming you are using) and it's easy to find those arguments on google. 2GB is a minimum for smooth running in minecraft 1.8
Thirdly you can turn vbo's on in the minecraft video options.
Try those and see how it goes. If you still have issues make a support post on our forums (www.pixelmonmod.com)