Hello,
1) Real time mode is designed for live communication scenarios.
2) Reduce the total bitrate of the videos that you are trying to play. Use CBR rather than VBR content. Review your D3D swap chain code to make sure it is not causing the bottleneck. Use Visual Studio to see if you can identify the bottleneck.
Analyzing the performance of Windows Store apps
http://msdn.microsoft.com/en-us/library/hh696636.aspx
I hope this helps,
James
Windows Media SDK Technologies - Microsoft Developer Services - http://blogs.msdn.com/mediasdkstuff/