Question 4K 60 FPS lag on youtube

[Rally0078]

Everytime i try to play a 4K60FPS video on Youtube, the video stutters bad and eventually stops and only the audio keeps running.
PC spec:
CPU: Intel i7 6700K
GPU: Nvidia GTX 660 2 GB
I use Firefox. There is no stutter when i try to play 1440p60FPS. I have a decently fast internet(100Mbps download speed)
Is it because of my GPU that can't do hardware decode 4K60FPS?

 

[Aeacus]

3840x2160 at 30Hz or 4096x2160 at 24Hz supported over HDMI. 4096x2160 (including 3840x2160) at 60Hz supported over Displayport. Support for 4k tiled MST displays requires 326.19 driver or later.

Source: https://www.geforce.com/hardware/desktop-gpus/geforce-gtx-660/specifications