The quick benchmarks in this article are of the Haswell-based MacBook Air released a few months back. The system has the Haswell Core i5 with HD Graphics 5000, 4GB of RAM, and 120GB SSD. The benchmarks I've uploaded to OpenBenchmarking.org are comparing OS X 10.8.5 to OS X 10.9.0 while using Xcode 5.0.1 with its Clang 3.3 SVN / GCC 4.2 compiler stack.
To see these OS X 10.8 vs. OS X 10.9 performance benchmarks see the 1310285-SO-OSX109MAV15 result file. Overall, there weren't many performance changes found in our usual performance tests run natively on OS X, but there was some small OpenGL changes and a few other areas that benefited from Apple's Mavericks release.
Apple advertises 200+ new features to OS X Mavericks from new and improved applications, expanded OpenCL support, compressed memory support when under memory pressure, SMB2 is now the default file-sharing protocol, application-layer VPN support, timer coalescing support, and much more.
More OS X 10.9 Mavericks benchmarks are forthcoming from other Apple hardware.