This is the important bit:

When numerous instances of Duff's device were removed from the XFree86 Server in version 4.0, there was a notable improvement in performance.[3] Therefore, when considering using this code, it may be worth running a few benchmarks to verify that it actually is the fastest code on the target architecture, at the target optimization level, with the target compiler.

Optimization without testing is worse then pointless. It's premature and we know that's evil.