In my case I was using sonarr/radarr, and plex. When download is finished and sonarr was copying files to destination location - plex is starting indexing it and probably both process needs a lot of computation. Im disabling plex (during copy process) and monitoring it. Probably I will disable auto scanning folders in plex and add sonarr plex integration if it will fix the issue. Stay tuned or ping me for further updates.