I performed a lab under these circumstances and there was not lagging; however, I noticed that the CPU Usage went up in Opera,Google Chrome and Firefox up to 100% and like you mention the CPU Usage in Microsoft Edge stayed around 20%. Base on your case I performed a research and I found some helpful links:
The content on the above site is not controlled by Intel. This information is offered for your convenience and should not be viewed as an endorsement by Intel for the merchants or services offered there.
Hope that helps.
Thank you for your information. With further testing, I can play youtube 4k video on Edge browser with no frames dropped but sure not on others browsers.
I think that the reason of high CPU loading on the other browsers is that the video decoding are processed by CPU instead of "GPU-accelerated decoding". When CPU usage go up to 100% in Opera,Google Chrome and Firefox, the CPU processing power is not enough and therefore video playing have frames dropped and result lag.
Hope that there will be new drivers come out soon for adding support for other browsers video decoding with GPU.