Readahead is a system call of the Linux kernel that loads a file's contents into the page cache.
[1][2] Many Linux distributions use readahead on a list of commonly used files to speed up booting.
This will make additional installed services start faster, because they are not included in the default readahead list.
[3] In Linux distributions that use systemd, readahead binary (as part of the boot sequence) was replaced by systemd-readahead.
[4][5] However, support for readahead was removed from systemd in its version 217, being described as unmaintained and unable to provide expected performance benefits.