This is simply the Outlook account of the user communicating with the email server to synchronize the data that is displayed on the users' computer in Outlook. This may occur when the connection is slow, when the email box of the client has become so large that it needs the extra time to synchronize, when the email server has become slow and sluggish due to a lack of hard drive space on the server and other reasons.
Since you say that only one user is experiencing this problem, you could start by having the user clean out old mail. Deleted items folder should be emptied of all items. The Inbox should not have thousands of messages sitting in there and perhaps would be a good time to institute Archiving of old items, say older than 6 months to a year old. Make the default Archive location their home directory on the server for backup purposes as the default is on C:\.
If this does not improve the performance in Outlook for the user, check his\her connection to the server. Is it on a 10\100 network card and is it able to connect at 100 if others are able?
Does the user computer have many programs running that are connected to the server? Always a good idea to run a disk cleanup and defragment for good measure.
HTH