Microsoft has just updated the following KB Article:
Distribution point installations or upgrades may take longer than expected in System Center 2012 Configuration Manager
Symptoms:
On Microsoft System Center 2012 Configuration Manager sites that have many Standard or Pull distribution points, installing or upgrading all distribution points may take longer than expected. This can occur if the Distribution Manager component cannot create additional threads for the installation or upgrade process.
Additionally, you will receive messages that resemble the following in the Distmgr.log file:
DP upgrade processing thread: No more available threads left to process any more upgrade distribution point notification. Will wait for existing distribution point upgrades.
If you receive this message repeatedly, you can reduce the overall time that is required to complete the process by increasing the number of processing threads.
Full details at:
https://support.microsoft.com/en-us/kb/3025353