That's a somewhat misleading comparison even for the time. All his C++ files could be compiled in parallel and recompiled incrementally, and his need for -flto to get good performance is really just due to how he structured his program. He seems to regard cpp files as the default location for functions and inlining as an optimization, but IMO it's the opposite. An empty function goes in the header, and every function starts off empty. You move it to a cpp file once it requires including a header. Moving a heavy function from a header to a cpp file is an action to optimize the compile time of your program.

On the other hand, it could be worse. There are some monstrously slow template libraries out there, and he's not using any of them. His code isn't very optimized, but it's sane.