[Ace17]

> IIUC, the 20% slowdown happens with a build that does nothing, it's just the scons overhead.

That's the whole point of performance measurement for build systems. The overhead is precisely what counts here.

If you have many source files to recompile, the fraction of time spent in the build system itself is going to quickly drop to almost zero : build-system stuff is generally several orders of magnitude faster than compiler stuff.

So it would make little sense to compare build-system performance on full builds.