I should have made it clearer that we expect to release a new installer variant for macOS 11.6 Big Sur that supports both Intel and Apple Silicon architectures later this year (i.e. in several months) when Big Sur releases. It will be much easier to support newer optimizations in that variant.  We are in the process right now of getting builds to work on the developer previews and on developer hardware. We will look at optimizations for that variant then.

Please drop the idea of trying to change how we build on 10.9 (and, yes, we are perfectly capable of finding newer compilers to run on 10.9 but that's not the point - we *only* support building installers with standard Apple Developer Tool chains and with good reason); hacking on 10.9 is not worth it at this point.