Why GNU grep is fast (2010)

GNU grep uses raw Unix input system calls and avoids copying data
after reading it. So instead of using line-oriented input, GNU grep reads raw data into
a large buffer, searches the buffer using Boyer-Moore, and only when
it finds a match does it go and look for the bounding newlines. Finally, when I was last the maintainer of GNU grep (15+ years ago…),
GNU grep also tried very hard to set things up so that the *kernel*
could ALSO avoid handling every byte of the input, by using mmap()
instead of read() for file input.

Source: lists.freebsd.org