Right now dev-util/boost-build ebuilds unpack the entire boost distribution whilst only using a tiny proportion to build the necessary build utilities. Especially on slower systems more time is spent unpacking the distribution than actually compiling. It would be nice if the boost-build ebuilds would do something similar to qt4-build.eclass and unpack only a subset of files.
This has been fixed long ago in >=dev-util/boost-build-1.35.0-r2.