Because Outlook with the KOC is in a full online mode, it does not have the ability to store data in an offline cache. So, while scrolling Outlook must cache (in memory) information in real-time. During the pauses, Outlook is sending an IMAP "FETCH" for anywhere from 40 to 50 messages. It is downloading the headers for those messages, then can resume scrolling.
Unfortunately, there is not a better way to do it without an offline cache. Trying to read all of the headers when clicking on the folder would create an unacceptible delay going from folder to folder. Also, a very large folder would cause an impossible delay. Perhaps some way to cache in the background would work, but even that would cause memory usage problems for very large folders.
The feature for an offline cache is filed with us as a key feature. There is no ETA for this feature at this time.