Do to some Synchronization logs failures it will create thousands and slow down your Outlook. 

Step 1. Turn off the download logs by 

Opening outlook go to FILE, then Account Settings, Then Data Files tab, click the user account and settings, then Advanced Tab, uncheck the download shared folders.
     

Step 2. 

Delete synchronization logs

Find a syncronization log failed email and create a rule base on that email based on the subject and move to deleted. Run the rule and Empty the deleted folder.