When running Nexuiz at 1280 x 1024 with high quality settings, the Core i3 2100 nor the Core i5 2500K was fast enough to provide a playable experience for this once popular first person shooter.
The Xonotic performance would be even lower. For what it's worth, with this unplayable Nexuiz demo the Core i3 2100 with its integrated graphics is at approximately 85% the speed of the Core i5 2500K.
With OpenArena at 1920 x 1080, the Core i3 Sandy Bridge CPU that's $100 USD less than the Core i5 2500K was only 8% slower. It was still very much playable at this HD resolution for the ioquake3-powered game.