If you've wanted to know more about why high item counts and restricted view requests can impact the performance of your Exchange environment, we've just released some detailed information about th...
Updated Jul 01, 2019
Version 2.0
Blog Post
While I don’t really cover cached mode in detail in this topic, the issues you see in online mode are still there in cached mode, they are just shifted to the client (for the most part). If you’re client machines are not robust, then you may see significant
performance degradation with high item counts. This paragraph touches on the issue with cached mode and item count:
If you are running in cached-mode, (the default mode for Outlook 2003 and later versions), then client performance can be an issue as the number of items in a folder increases. One thing that you should do is keep your OST files (the local data cache) free
of fragments. You can use the Windows SysInternals tool Contig for this purpose. To download the latest version of Contig, see Contig v1.54.
I’m going to cover this in a little more detail in the Planning for Large Mailboxes with Exchange 2007 white paper that I’m hoping to release in July. In the interim, Matt Gossage has some good information in his Optimizing Outlook 2007 Cache Mode Performance
http://msexchangeteam.com/archive/2007/12/17/447750.aspx