There may be some slowing down in the individual thread's clock speed. But from mid-30s and on, the number of threads multiply incredibly. You start running ~24 threads instead of just 2-4 threads you had before. When dropping a single feature into a codebase, a zillion considerations ranging from legibility to backwards compatibility, from maintainability to efficiency get run in a zillion threads. Your vision becomes larger, you can see bigger problems, enhancements, systems and you can actually implement them yourself. But that takes time. After all, you are just one person. That's why the mix of senior developers with junior developers works great. Of course, there are some senior developers who can just take a feature on single-handedly and run to the finish line too...