Raspbian is the Debian Linux distribution optimized for the ARMv6 Raspberry Pi. Older versions of Raspbian are based upon Debian Linux 6.0 on the Linux 3.1 kernel and GCC 4.4.5. However, the latest Debian Linux 7.0 on the latest Raspbian package-set has the Linux 3.6.11 armv6l kernel and GC 4.6.
A Phoronix reader and long-time user of the Phoronix Test Suite and OpenBenchmarking.org, Bryan Quigley, has some before and after benchmarks of Raspbian on the same Raspberry Pi board. Making things interesting, he also compared his results to his Intel Pentium 4 system clocked at 2.80GHz for its single-core with 512MB of RAM and other ancient specs while running Ubuntu 12.04.
While the old Intel Pentium 4 is still able to outperform the Raspberry Pi for most workloads, the Raspbian Debian Linux performance has improved a great deal when encompassing the Debian Linux 7.0 packages.
Embedded below are just a teaser of the benchmarks while all of the system details/logs/benchmarks are available from his OpenBenchmarking.org result file: 1302242-BY-1205272AR49.
Bryan has also blogged about the performance. "Take aways: Performance has improved greatly. Still get’s crushed by a P4, but performance per watt is now clearly in the Pi’s favour."
Through the MONITOR= environment variable he was able to monitor various system vitals as well during the testing process. See the Phoronix Test Suite documentation for more details on the abilities and options.