set fast-init-file

SET FAST-INIT-FILE ALWAYS/never/ask  -  whether to write .mmfast

With the usual ALWAYS setting, MM will maintain a fast format initialization
  file named .mmfast, based on the .mminit file.  MM can then start up 
  slightly faster.

If you change it to NO, you should also delete .mmfast if it exists, and 
  you will save the disk space .mmfast would occupy.  .mmfast is, however,
  a very small file.

You can also set it to ASK, MM will ask whether to update .mmfast if it
  finds .mminit is newer than .mmfast or if you use SAVE-INIT.  Normally 
  it is desirable for the two files to correspond.

For more help, type "help" and one of the following:
(at any prompt:)  save-init   set   show