Workaround
To work around this issue, run the following command in Windows PowerShell:
Set-csPresenceManagementState -MaxPublishersPerBatch 1500 -NotificationBatchInterval 2
Resolution
To resolve this issue, install the June 2015 cumulative update 6.0.9319.55 for Microsoft Skype for Business Server 2015 (Front End server and Edge server).