Thank you all!
@Stan: May be you're right above my purpose when I raised the question here. I searched on Internet but didn't find an expected answer so I want to confirm it. Why I thought that cause my SquirrelMail ran faster, but RoundCube was slow. Every other things were the same (Postfix, Dovecot, Mysql, Apache) so I thought it might due to the more processing and resources for RoundCube. The Email server's still in the development phase, so the load is too few. Maybe I should consider to check in those software module, if anything made it slow.
@p8x: Thank you for you realistic information. For your information, when I checked the SystemInfo, the Total Memory was nearly 1GB, but the Free Memory was only 50MB. When I tried to stop httpd, mysqld, dovecot, postfix; the Free Memory expanded to 140MB. I also check the CPU load for those service, and I found they not take too much CPU resource.
Do you think any problem in my OS (Centos5)? I run only Centos5, SSH service and those email services. 50MB free memory maybe too small while the server's in work.
BRs/ HoanNT
List info: http://lists.roundcube.net/users/ BT/9b404e9e