LWJGL uses the OpenAL-Soft library for its OpenAL support.
One of the MESA developers, Timothy Arceri, while profiling OpenArena noticed that about 14% of the CPU usage was being used for processing sound. After looking into the situation he implemented SSE2 and SSE4.1 linear resamplers in OpenAL-Soft which in some cases have halved its CPU usage (or made it twice as fast).
The optimisation was included in the 1.16 release of OpenAL-Soft and as a result is passed onto LWJGL users.
Gotta love open source development.
The full details of the change can be read on Timothy's blog here