[Help] Plex codec for streaming to chromecast?

So i've got plex and xbmc (not kodi) installed on linux media server that hosts video files, all different containers and codecs. Looking to stream to my Chromecast, but not liking the results:

• when playing locally on the server via xbmc I get perfect video quality (as expected)
• when playing locally via plex I get mediocre video quality in FF but better quality on chromium

However, when streaming plex to chromecast I get terrible quality. Not a ton of stuttering or constant buffering - just things that point to a bad bitrate: low res/framerate, lower when there's any motion. (If there's water in the scene it's just laughable)

Any thoughts on this? I'm new to plex and I've read a few things that point toward the container being the problem - do i really have to go through and re-encode all my video files? or is that just a red herring? TIA....