Why is it transcoding?

[Volts]

@number8.1

On 1080p movie I confirm there are spikes closed to 100Mb

You can see the bitrate of the media itself if you Get Info and View XML in Plex, and look at requiredBandwidths="7303,4932,3415,3415,3415,3415,3415,3415 (example).

Streaming bandwidth

But you can also see that if your player device had a network buffer of 75 MB size, this peak at the begining of the movie won’t matter that much anymore. Those various numbers in requiredBandwidths are the required bitrates for 5/10/25/50/75/100/250/500MB network buffer sizes.

Note that when you play video, it will be transferred over the network as fast as possible, in gulps. The client will download a big gulp of the video and fill its cache, then will take additional sips as it plays.