I don't think I've properly explained the idea... The goal is to get the same performance as ninja, in the common case when the dependencies/build steps haven'

I don't think I've properly explained the idea... The goal is to get the same performance as ninja, in the common case when the dependencies/build steps haven't changed. We somehow use the makefile to generate the ninja file; likely by executing make and capturing the dependencies & commands (without executing the build commands). But we don't encode the full complexity of the makefile in the ninja rules; if someone adds a file or changes a flag or even adds a #include directive, our ninja rules will be out of date. So we are able to run the build entirely in ninja, but we may not be building what the makefile would build. So we have to 'waste' some CPU time running the make -> ninja conversion in parallel to verify that our ninja build is still correct, but as long as 'total ninja build time' > 'make to ninja conversion time', we come out ahead in wall clock time.