To get the best performance under Linux, especially on SMP systems, the use of NPTL is advised. The difference in performance can be over a factor of ten in some circumstances.
NPTL is the default library on modern Linux distributions, so there is generally not a problem, except if you use a statically compiled version that, for portability reasons, defaults to LinuxThreads. This includes all .deb and .rpm files provided by us up to and including 2.9.18.
When running a PowerDNS-provided static binary of 2.9.18 or lower, it may make sense to recompile, or to upgrade to a newer version, if available. When recompiling, be sure to use a supported compiler, like g++ >3.2. You might also consider moving to a distribution supplied version.
A good indication that your installation might benefit from such an upgrade is to watch the 'cs' count in the output of vmstat 1. If this is very high (> 10000), you are suffering from a LinuxThreads performance problem called 'overspin'.
Thanks are due to L. Bunt Jackson who noted the static compilation problem in an article in Dr. Dobb's Journal.