Consider the following scenario:
Your Lync 2013 (Skype for Business) or Skype for Business 2016 client has the Personal information manager (PIM) feature enabled as follows:
You have 10 or more nested Contacts folders in your Outlook mailbox. These are user-created subfolders under either the main Contacts folder or the main folder level (the top of the Information Store).
In this scenario, the Lync 2013 (Skype for Business) or Skype for Business 2016 client consumes excessive CPU resources and becomes unresponsive.
By design, Lync clients try to cache and synchronize all Contact folders. When the number of Contact folders exceeds 10, this may trigger the issues that are described in the "Symptoms" section.
To resolve this issue, reduce the number of nested folders under the main Contacts folder in Outlook. You can redistribute these nested folders among fewer than 10 parent folders or move them to a .PST folder.
The following PowerShell commands help identify the number of nested Contacts folders in Exchange or Outlook for a specific user:
PS C:\PS> (Get-Mailbox | Get-MailboxFolderStatistics -FolderScope inbox).count
PS C:\PS> Get-Mailbox | Get-MailboxFolderStatistics | Select Identity