Outlook and updating increases
This means that in the worst possible scenario, an update to the Address Book won’t become available to the user until 48 hours after the change.This situation however is extremely rare since the default maintenance time on Exchange is at AM and updates and downloads are usually performed during working hours.If your case is either, the effective method is to reduce the number of PST files opened in Outlook.
In that case the whole OAB is being downloaded again.
You can reduce the memory cache size which Outlook assigns to each PST files.
Take the following steps: Nevertheless, if you don’t have multiple PST files connected to Outlook currently but you still encounter the issue, it is more likely that something goes wrong with your PST files, such as small errors or corruptions.
Prompt before for full download As the OAB can be quite large in some environments, it could be wise to be prompted when Outlook wants to download the full OAB.
This allows you to abort the update process when you are for instance on a slow or expensive connection.
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As a rule, when you open a PST file in Outlook, Outlook will allocate a certain part of memory to it as a cache to improve the performance.