Tail also doesn't always need to read its entire input. When given a file path, it can read backwards from the end of the file until it has found its n lines. That makes tail fast on huge files (as long as they have normal line lengths), but it also complicates the code.

And you may want to mmap the file, rather than open it. Whether that is a speeds up things depends on OS, OS version, file size, file system, available memory, phase of the moon, etc.