In general, best performance is achieved on recent Linux 2.6 kernels and using MySQL, although many of the largest PowerDNS installations are based on PostgreSQL. FreeBSD appears to achieve lower packet rates both for the PowerDNS recursor as for the authoritative nameserver, this is still being investigated. No comparative measurements have been done for Solaris installations.
On Linux, make sure to read Section 9.2.
Database servers can require configuration to achieve decent performance. It is especially worth noting that several vendors ship PostgreSQL with a slow default configuration.