This article is just to provide some initial reference of performance expectations for Nouveau on modern NVIDIA "Fermi" GPUs with the upcoming 13.10 release. Ubuntu 13.10 still has yet to pull in Mesa 9.2 and it will also be switching over to using Mir/XMir for the desktop, but for where things stand today, we have some new benchmarks at Phoronix. Ubuntu 13.10 may also still pull in the Linux 3.11 kernel. More thorough benchmarks from a diverse range of hardware will come once all these changes have settled.
The benchmarks today happened from the ASRock Vision 3D NetTop with Intel Core i3 370M CPU and NVIDIA GeForce GT 425M graphics. The Nouveau driver still lacks Fermi/Kepler re-clocking support (even with the latest Linux 3.11 kernel code) so the GT 425M is still handicapped to its boot frequencies of 202/324MHz rather than 560/800MHz.
Full system details and the benchmark results in full can be found on OpenBenchmarking.org within 1307116-SO-ASROCKVIS42.
The Nouveau driver also only supports OpenGL 3.1 at this time rather than OpenGL 4.3 as exposed by the proprietary driver.
Obviously with the clock speed problems, which won't be resolved until at least the Linux 3.12 kernel but there's yet to be any Fermi GeForce 400/500 re-clocking code to emerge, the open-source Nouveau driver's "out of the box" performance will continue to suffer.
See the rest of the results here.