Page "MH Message Handling System" Paragraph 11

The performance of MH is very much dependent on the file system and other features of the operating system that it runs on.

Every read of a message will require starting a separate program, a directory scan in a large directory and a file open before the data can even be accessed.

On the other hand, mailbox formats such as mbox which use a single file often require insertion or removal of data in the middle of the file when messages are read or altered and this can be a very slow procedure.

In these cases MH will be much faster.