[imp] Performance problems when sorting mails

[Pablo Guaza Peces]

Hello there!
I'm getting a little crazy looking for a way to solve this:
I've installed this software in two HP Proliant 4x3.2 GHz. Xeon Dual 
Core with 12GB RAM, with Red Hat AS 4, with a load balancing switch:
Horde 3.1.7
IMP 4.1.6
Turba 2.1.7
Ingo 1.1.5
Apache 2.0.52
PHP 4.3.9

When I sort by date all the emails (about 2400) that i've got in my 
Inbox it takes about 3-4 seconds. The thing is that I'm the only user 
accesing the system, once all users (about 5500) are allowed to use the 
webmail this will probably increase (to death? ;-) )

I've installed the ionCube PHP accelerator, and performance has 
increased a little bit, but just a little bit.

The servers load is really low when doing the sorting, almost 99% idle.

Is this usual? Is there a way to get a better performance? Is it a 
matter of PHP tuning? Any clue? Too many questions? :-)

Thank you!!

-- 
Pablo Guaza Peces
Tecnologías de la Información 
Unidad Técnica de Infraestructuras de Equipos Centrales
http://rincon.uam.es/cgi-bin/v2/dir.cgi?cw=658050537109375