Quite a bit of code is compiled with toolchain compiled within build system. This would need to be somehow distributed to distcc slaves (nfsv4 ?) and taken advantage of, maybe even some parts of toolchain building process could be cached/distributed, but many distros specifically disable ccache/distcc on those packages.
Another thing to take care of for distcc setup would be disabling LTO - linking is low on our machines (Xeon with x4x chipset) and cannot be distributed which makes distcc idle for that time. Disabling LTO pretty much makes slaves work all the time (test setup had c2d as master and xeon x4x as slave)