IMP stays slow with utilizations

[Lista IMP]

dear sir,


  i've noted that for new users, my imp is very good and fast, bat when the 
size
  of the mailboxes of the users grow up, the program stays very slow to 
deal with
  the e-mails.

  i think that this problem happens because the size of the mailboxes grow 
up. and

   they are managed by imap that let all the mailboxes in the server and 
have to
  download all of then when someone access it.
  (some mailboxes like sent-mail have almost 100Mbytes)

  my question is:
  -is the size of the mailboxes the problem that slow down my connections?
  - if it is the reason, there is a way to me bypass this problem, maybe 
changing
  imap by pop3??

  does someone other reported this same problem?

  p.s. my connections are done from inside a LAN (10Mbps)
  the versions of the programs are:
  horde-1.2.2-2cl
  imp-2.2.2-1cl
  my linux is conectiva linux 4.2

  sincerely,

  Alexandre Nuernberg
imp@help4u.com.br