I think the build guide should mention using LTO for release builds.

In return for far slower link times (so this is not something that should be enabled during development), I got a big performance increase from LTO, for example with Cornfield Crossing in Supertuxkart going from 46 to 51 fps, an increase of about 11%.

By the way, even when LTO is enabled, the "This may take a while" warning on the build guide seems to me a bit pessimistic about compilation time - I first assumed it would take closer to three hours than 20 minutes.