The Roku has very limited codec support - for video it only supports H264, and not at high bitrates. So if you have MPEG2 (ripped dvds, some bluray) MPEG4 Part 2 (xvid, divx), or VC1 (bluray and hd-dvd) it HAS to transcode it. Same with a bluray quality bitrate on H264 - it can't handle it and needs to be compressed.
It also doesn't support AC3 or DTS unless the Roku is plugged into a receiver. I bet Plex tries to transcode them both into AAC.
So what does all that mean? It means every video you play on the Roku will hog a lot of CPU power on the host computer.
shmenguin wrote:how is it with uncompressed MKV bluray rips?
Depends on what codecs the device you plan on playing it supports - the apps themselves for different devices don't add codec support. So for example if you loaded the Plex app on the Sony Google TV bluray player you should be able to play everything, since the bluray player had to license all the codecs for playing back blurays. If you tried doing it on the Sony Google TV television set you wouldn't get DTS audio and Plex would convert it to Dolby Digital on the fly.