which mailbox folder is the best?

  Hello Mail-Box Users, hello Mark Overmeer,
im just renewing our webmail and asking myself if the system would get a 
performance boost
if I changed the mailbox folder type from mbox to mh or maildir?
After reading the online docs I am not sure about that.
My users have at least 20 MB in their files and opening those takes 
quite some time on our Server.

Perhaps someone could give me a hint or share some experience.

Greetings from Kiel, Germany (where it is always raining in the summertime),
Simon.