Plex new Transc 90-99% watching the video.

Hello.
Someone tell me where to look for a solution?
Synology RackStation RS2414 + it installed Plex Media Server
Through appleTV displayed on the Samsung TV.
The problem is that when viewing large files (15-20 GB) for a long time to boot, and then maybe even hang the picture.
If watching films is small in size, it all works well.
If you open the video of any size (2-30 GB) of the resource monitor on RS2414 + shows the CPU load of almost 100% (process
)
All LAN operates at GB and movies from the Internet 4K resolution displays without delay.