>Less lines of code is better
Too simple of a way of looking at things.
GNU Cat actually performs better than Busybox Cat because it does actual buffering and mmap trickery so it can handle large files better.
Also Busybox has no handlers for special cases like multibyte strings, unicode and other obscure encodings and whatever. Many types of edge cases are just simply not handled in Busybox for the sake of minimalism.