This has got to be one of the most subjective matters, ever - How fast/slow is Outlook performing. While researching another topic, I came across a blog post from the Exchange team which allows us to give our users guidance on how to structure their mailboxes and why. Check it out here:

http://msexchangeteam.com/archive/2005/03/14/395229.aspx

This is an older article, however this article is also being referenced in the Exchange 2007 deployment guides. Remember that Exchange 2007 still uses the ESE engine, and as such a lot of what we've learnt over the years still applies.

In summary: Critical top level folders should have no more than 5000 items max. These include, Inbox, Sent Items, Calendar, Contacts. after this, create sublevel folders. If your mailbox is over 2GB's, switch off Cached mode, and try to limit the number of apps on your machine that use MAPI, search engines fall into this category as well.

Exchange 2007 Messaging Records Management can help lessen this load, but make sure you understand the implications around this topic. Messaging Record Management is a nice change from the old Mailbox Manager, however some features require Outlook 2007. Check it out here: http://technet.microsoft.com/en-us/library/bb123548.aspx

With enough memory, Exchange 2007 can reduce the number of read/write ratio's to 1 read for every 1 write. This directly affects things like sort speed, retrieval speed, folder rendering and changing views, etc. This makes a really good argument for examining your disk infrastructure and how your disks are arranged - RAID, stripe, JBOD, etc. Knowing that a lot of Exchange Mailbox servers at Microsoft use RAID 10 configurations, striping is SERIOUSLY back in fashion - if you can afford it!