On GFXBench, the leaked results consist of T-REX HD C24Z16-Offscreen and Egypt HD C24Z16-Offscreen results. This is compared to Sony Xperia Z1, LG G2, Samsung Galaxy S4 LTE-A and Apple’s iPhone 5/5s :
Other than the off the charts results from Galaxy S4 because of its higher clocked CPU and “cheated” tweaks to the phone, the other devices scored similarly. Nexus 5’s performance is not far away from iPhone 5s’. Additionally, this Nexus 5 is probably an engineering unit, meaning that performance should be better when launched.
On the other hand, it seems that Nexus 5 scored lower in AnTuTu 4.0. Nexus 5 only scored a score of 23,345. More optimizations to the released device should increase these scores.