Zone Builder runs at full speed on my crappy laptop from 10 years ago -- the system requirements just aren't that high. Assuming you're running it natively in Windows, you could try to close all other nonessential processes while using ZB and see if that helps. If it doesn't, you're probably just going to have to buy better hardware. It's hard to say for sure without knowing what CPU you have, how much RAM you have, or what graphics card you have (if any).