I have KMail 1.9.1. My mail is POP.

I have SpamAssassin filtering my incoming mail.

While all my mail folders have grown significantly (the spam folder has over 3900 messages), KMail has become gradually very slow. It's slow not only for downloads, but for simple actions like deleting a message or moving it (sometimes moving a message to another folder can take one minute). There are moments when, apparently, some process ends and KMail is able to behave normally, like before.

The rest of KDE has a decent performance, as usual.

When I switch to KMail, my recently installed Ksensors shows a sudden increase in the CPU status, with "user" raising from about 3% to 80% and "system" from 0.8% to 20%.

Here is the output of hdparm /dev/hda

I will appreciate some hints about this performance problem.
I followed the advice on the KMail manual about how to optimize my filters, but that did not make any difference.
I suspect that SpamAssassin could be the main culprit but, why does KMail bog down long after the messages were downloaded from the server?

What format is you mail stored in ? Maildir or mbox ?

You might wanna google around a bit on this. There are several articles on the web that present a detailed comparison between the two.

i recommend switching to mozilla

Thank you both for your reply. The default format of KMail is Maildir. I'll look into the differences and will post again.

Switching to another mail client is certainly an option. I like Firebird, but for the moment I like the integration that Kontact gives me, and the fact that I can synchronize with a PDA; I don't think Mozilla Firebird gives me synchronization options currently (correct me if I'm wrong).

Before I switch, I would like to find out if there are ways to optimize KMail (I worked on my filters already).

OK, here is what I did wrong: I did not compact my folders on a periodical basis. The ghosts of the deleted and moved messages persist until you compact the folders (purge them). That adds an unnecessary weight to the program, thus slowing it down.

I should have RTFM and learned this simple rule: compact the folders frequently.

It took me a few seconds and now KMail behaves well, just like before.

By the way, on my second post on this thread I typed "Firebird". I meant THUNDERBIRD. I apologize for that. I like that client a lot, and I feel tempted to switch. But as I said, I got used to KMail (so simple and totally integrated with the rest). Anyway, thanks for bearing with me, folks.