Meaning, slightly, better performance, Nick Piggins dentry scalability work has been merged, improving certain workloads up to 35% in simple tests performed by Linus Torvalds.
The next generation filesystem BtrFS is receiving updates which are expected to improve performance and stability. Hopefully making it possible to soon install Ubuntu on BtrFS in less than 3 hours.
The memory system has gained support for transparent huge page which should transparently improve performance by up to 10% in some situations.
The drivers have also seen a number of updates as usual.
The always great Linux Weekly News has more details on 2.6.38