KMail and Archiving

Hi I'm using KMail, KDE 3.2.3 and SuSE8.1. My mail folders are now starting to get very large. Some have more than 2,500 messages and I'm beginning to think that I should archive messages older than some date. However, is it necessary to do this? Does having so many messages slow the operation of KMail? I can't say I've noticed a slowing down but then the folders grow very slowly. If archiving is recommended then I'd still like to be able to access the archived folders/message with KMail in the easiest possible way. Is there are recommended way to do such archiving? Cheers John Satherley