[Dovecot] mail open slowly

Hello,

I have a dovecot IMAP server (relative small hardware: HP Microserver with 6 Gbyte RAM, linux soft RAID1 with 2x 7200 SATA disk) with 100 Gbyte maildirs.

Server works fine but sometimes I can open mails slowly (5-10 sec), typical with new mails.

It's intresting when I open an another old mail (with 0 sec wait), after new mail open quickly.

Is it dovecot mail indexes or I/O problem?

Load and CPU use is small typical 0.10, 10%, I see small IO wait.

Debian 6.0, Dovecot 1.2.15, fsync and nmap is disable

Best Regards,

Gabor