I tab-cast from my desktop system, i5-3570K 4.2 GHz. It still stutters slightly, no difference at 720p or 480p, even though the overall CPU utilization stays low. I keep retesting with each new update of the Google Cast extension, but there hasn't been any improvement.
Plex can cast the same video stream to the Chromecast from the same desktop computer without stuttering. It's just poor coding by Google.