[imp] Slow show message when the mail is big

[HardWING]

Hello,

I found it takes so long time (more than 4mins?) to open the mail before show the message if the mail is too big (4MB size inline text/html content).  How can make it faster?

I am using:
IMP H3 (4.3.4)
Dovecot for IMAP and Maildir for mail storage.