Best optimizations for Plex Home Streaming? Do You need good internet for high quality shows or movies? Transcoding good or bad? Should I use Direct Play.

I'm really struggling here, I'm new to this and its all very confusing. The most glaring issue for me though is the low quality on home streaming. I'm streaming 8 Mile Blu-ray UHD to an HD TV. Just cant get the quality to where it should be. Is it because of my home network or my Plex client settings. I'm using the Plex app on Vizio. Please help.