Wikipedia
In computing, FASTOPEN was an MS-DOS TSR command, introduced in version 3.3, that provided accelerated access to frequently-used files and directories.
It worked with hard disks, but not with diskettes (probably for security when swapping) or with network drives (probably because such drives did not offer block-level access, only file-level access).
It was possible to specify for which drives FASTOPEN should operate, how many files and directories should be cached on each (10 by default, up to 999 total), how many regions for each drive should be cached and whether the cache should be located in conventional or expanded memory.
If a disk defragmenter tool was used, or if Windows Explorer was used to move files or directories, while FASTOPEN was installed, it was necessary to reboot the computer afterwards, because FASTOPEN would remember the old position of files and directories, causing MS-DOS to display garbage if e.g. " DIR" was performed.
The "fastopen" name has since been reused for various other "accelerating" software products.