The recently released R2 version of Configuration Manager 2012 has suffered from slow performance while downloadning in WinPE. See this thread for more information:
The problem – according to the thread above is the refresh rate of the progressbar. A hotfix has been released that solves this issue as well as an issue for PXE boot points on site servers.
See http://support.microsoft.com/kb/2905002 for more information and to download the hotfix