As for items that say... "This item is not supported because it is a..." The reason you see this is because only video and audio playback is supported at this time. These things are being enabled at this time in upcoming releases of the plugin, as most of it is not yet supported. Good news is our buddy Frank has already got about half of it working in his beta.
As for slow playback, listen carefully... Just like any web browser, if you browse and playback movies from videofriender or whatever, especially on the weekends, ESPECIALLY if they are very popular movies to stream this moment, you will experience buffering, as these websites servers are constantly bombarded on weekends in particular. The week is not so bad, but unless Videofriender or whoever you are streaming from upgrades their network services and bandwidth, you will not like playback of movies from certain websites during the weekend. Nothing we can do about websites offering too little bandwidth to serve their users. Eventually, a download option will be offered in Navi-X for boxee so you can start a DL, then come back to a un-interrupted movie.
Remember as well, sites like MegaVideo who have a 72 minute playback limit, STILL have limitations in effect... so essentially, if you rack up what Megavideo site considers to be 72 minutes of video, you will have to wait an hour, then attempt playback again! Yes, it's annoying and no I'm not God or I would make other peoples servers work like a Ferrari.
So within all the power and glory that is Navi-X, even we are mortal and are subject to the environment around us. There is sooooo much to inform people about the Navi-X concept, but I will try to explain more as time progresses.