There's only a modest increase in performance between the 2013 MacBook Air and the 2012 MacBook Air. Depending on which models you compare, you can expect between a 3% and a 8% improvement in performance.
Considering that last year's update brought an increase of 20% to 25% in performance, this is a small step up. The reason for the minimal increase in performance is Apple's focus on battery life. The Haswell processor improves performance by 10% and uses 25% less power; however, Apple reduced that effect by switching to processors with lower frequencies. The low-end Air has been dropped from 1.7GHz to 1.3GHz and the high-end Air has seen its frequency dropped from 2.0GHz to 1.7GHz.
Focusing on power usage instead of just focusing on processor performance leads to a massive increase in battery life for the new MacBook Airs. Apple claims that the new 11-inch MacBook Air has an 80% increase in battery life, while the new 13-inch MacBook Air has a 70% increase in battery life.
Take a look at the benchmark results below...