I've tried this feature and it's not working as expected. I am using macOS Sierra 10.12.6 on a MacBook Pro 15 i7. The project that I'm working on has many plugins and currently is taken about 10 seconds on my computer to recompile. I thought that using Assembly Def Files could reduce that time in a couple of seconds at least. I followed the instructions and I've added the asmdef files to each directory of each plugin (starting from the most complexes) but the compilations times are exactly the same 10 seconds.