It is simply a FlatFile containing all Emails. A blank line followed by a line starting with From (ie "\n\nFrom ") is considered to mark the start of the next Email. Of course, this means that Emails containing a line that starts with From have to be mangled -- the convention is to prepend a greater-than sign, making it >From .
MBoxes do not scale. The entire file must be read sequentially in order to retrieve all messages, because there is no indicator to let a program skip over message bodies as it reads across the file. Many programs, such as the PINE MailClient, take the easy way out, and simply slurp the entire file into memory to break it down. If the MBox grows larger than your memory, you're going to be waiting for a while.