I was just experimenting with a Roku 2 and Plex, comparing to the Chromecast. The Plex media server running on my desktop PC can transcode external video streams from a wide variety of sources (e.g., Youtube, most U.S. TV network sites) and stream them to the Plex client on the Roku 2 via WiFi, just as tab casting does for the Chromecast. The Roku 2 is in the same location as the Chromecast and it's almost as compact as the Chromecast. They both get Excellent WiFi signal strength.
So explain to me how the video stream cast from Plex plays perfectly smoothly on the Roku, while the tab casting to the Chromecast from the same PC drops frames and stutters noticeably. Maybe Google needs to hire some programmers from Plex to help them out!