I've heard that having shared libraries like glibc allows for commonly shared hotpaths to remain in the CPU cache, making it faster. Whereas in musl, since the binaries all have their own copies of procedures, they are more often kicked out of cache. I don't imagine it has a big impact on a container where you are only running one or two binaries. Perhaps in a desktop environment with hundreds or thousands of different programs running, it might make a difference.