The Windows 8.1 version of the Media Browser app has been update improving the performance of the app and reducing the server CPU usages. MP4 and AVIs now play directly without the need to transcode or remux the files which reduced the load on the servers. Also ac3 5.1 audio is supports, music playback has been improved and there are new language options. On screen data has been improved giving you information on codecs/resolutions and whether the file is being transcoded or playing directly.
You can find the app in the Windows Store for £2.99