Eh.. it's not just you too be honest. Last time I was on it I went from 1200 FPS to 40 FPS in a second and my laptop is three times more powerful than what you have.
You need to take into account with what's running in the background. Although you may have programs closed when running GunZ, you might find running services can be a resource hog (like Windows 10 update services). Something like IOBit's SystemCare can help you disable services that you don't need (and privacy, yay!). Things like Skype can be a resource hog when running in the background with it's insane P2P connections. You may also need to disable your Internet Security (doesn't apply to all. Most are hogs) or enable game mode if exists.
