We recommend setting your Plex internet streaming quality settings to maximum for the best experience. Not only does this ensure you receive the best quality picture but it also lessens the strain on the server itself.
on Plex Web go to the settings icon in the top right, then go to Quality on the left and set Internet Streaming to maximum
on most Plex TV apps go to the menu on the left and scroll down and select Settings, then scroll down to Video Quality and set Internet Streaming to maximum / original (exact word varies)
on Roku’s (and some other streaming devices) select your username in the top left hand corner, select Settings, scroll down to Video, set Remote Quality to Original
on iPhone and iPad select the hamburger menu ☰ in the top left next to Home. Select the gear icon, Quality, Remote Streaming, show all, and set to maximum
You should only need to do this once for your account, though it certainly doesn't hurt to check this setting each time you sign in on a new device.