Plex would be dependent on the caching and throttling settings you have set in CloudDrive. Plex might influence the amount of data pulled based on your "Transcoder default throttle buffer" value and bandwidth limits in Plex Server settings. If I'm attempting to play cloud videos on Plex, I do notice an additional 1 to sometimes 5 second delay before the video starts, but I see similar latency in the cloud disk access times. Plex can't start playing the video until it buffers enough from CloudDrive. In my experience, Plex will fill the transcoder buffer to the limit specified, and request as many CloudDrive blocks that equates to.
Edit: I am on 1Gbps fiber though with a decent server; I do limit remote users to 4k 40mbps.