As has been pointed out, transcoding with Plex is CPU intensive. Depending on the source format and the playback device, you might be transcoding both video and audio streams. At the end of the day, it is the video stream that requires the most CPU. Audio streams are easier to transcode. When you play a movie, you can go to the Plex server dashboard and it will tell you whether the media is being transcoded or direct streamed.
As an experiment, you might try playing the movie on different devices. If you have a playback device that will play the media without transcoding, you should see less CPU. Over the last few years, I have updated to 4K Roku's for playback as they can handle most h.264 and many h.265 streams natively.