The Plex Dashboard will tell if that is the case. Inspect it.
Immediately after starting playback, it is normal for the bandwidth to jump to maximum, because the client wants to fill its playback buffer as fast as possible. Bandwidth will later drop down as it will only be used to top up the buffer as the player is consuming it.
If however the bandwidth is remaining constantly that high, it points to an incorrectly muxed file, which is unfit for streaming applications.
See High Bandwidth via Direct Play on Windows 10 - #4 by OttoKerner
