You might want to take a look at the “Now Playing” section of the server dashboard to get an idea of what’s causing this. Even if your internet download speed on your iMac is 600 Mbps – that doesn’t mean you get that bandwidth between your iMac and the Samsung TV. This can especially be problematic if there’s some glitch in your network setup that’ll make Plex think you’re streaming to/from a different network using an indirect connection (e.g. check if the stream in the “Now Playing” section is labelled as local, remote or indirect). Similarly, many smart TVs only offer a 100 Mbps LAN port (which is not enough bandwidth for some high-quality / high-bitrate media).
As for the 2nd message… the “Now Playing” info should also show if/what part of your stream require transcoding and indicate if that’s too much for your iMac. Especially older models will easily be brought to their knees if you’re transcoding high-quality / high-resolution media.
