From the Internet Super Hero blog, there's some new benchmarks posted for using mysqlnd (the native MySQL driver for PHP) with the two different PHP MySQL extensions - ext/mysql and ext/mysqli.

During the development of mysqlnd, the MySQL native driver for PHP - an alternative to libmysql, a good number of new tests have been written. Let's see how ext/mysql and ext/mysqli perform when running the test suite. Please note that this is a snapshot and figures might be slightly different when you repeat the test runs: mysqlnd is under constant development and your setup is likely to differ from mine.

I won't spoil the results of the benchmarks, but it is worth mentioning that he includes the configure options if you'd like to repeat the testing on your own systems.