Why 4K video runs slow in new laptop?

Hi, I just bought an ASUS N551JK with nVIDIA Geforce GTX 850M, 8GB RAM and i7-4710HQ processor. I also bought a Samsung 4K monitor U28E590D in order to play 4K content.

- As for still images I can display 4K content in my monitor with nice quality but when it comes to video, there is a terrible lag in the image and sound.
- I have tried both display port and HDMI connection running 4K@60 and 59fps respectively.
- I've tried several 4K videos in Youtube and the result is alway the same. I also tried some movie files played with the latest MPC-HC and VLC, but same result

Any help will be welcome. I am really annoyed with the fact I'm not able to play 4K videos smoothly.I also raised a case to ASUS but your experience sharing will be great.

Cheers,
 
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