I bought a 4K Dell P2721Q monitor and connected it to my old late 2013 MacBook Pro 15" via a Mini DisplayPort to DisplayPort cable. It connected without problems and shows that I have 4K resolution and 60Hz refresh rate.
When I tried to play 4K video on YouTube in Chrome, it was constantly showing a "loading" indicator. I thought it was problem with my Internet, but playing the same video in 4K on the MacBook display works fine and smooth.
Then I tried playing 4K YouTube video from Safari, it was definitely better than in Chrome, but it was still choppy. Doing some other activities like browsing/scrolling tall websites seems to work OK, there is are no lags.
What could be potential problems to investigate? Is this possible due to bad cable (I bought one in the mid price range, it has clear 4K 60hz marking on it and there are user reviews which claim it worked fine for them)? I noticed that after several minutes of playback CPU temp rises above 80 degrees up to 90 and then returns back below 80 (fans start to work louder). Could it be due to some CPU throttling?
Anything else I should try to check?
Macbook details:
MacBook Pro (Retina, 15-inch, Late 2013), 2.3 GHz Quad-Core Intel Core i7, 16 GB 1600 MHz DDR3, NVIDIA GeForce GT 750M 2 GB, Intel Iris Pro 1536 MB
Best Answer
It's about the decoding codec. Chrome does not support hardware accelerated vp9 codec on Mac OS, which is required to decode any 4k video on Youtube. Which means when decoding the 4k video it's using your CPU's resource instead of the GPU. Good luck trying to decode any 4k video using a mobile CPU, it simply won't work, a 9900k might do it, but a mobile chip, nope. For safari it should support hardware VP9 decoding, however, your Macbook is so old I highly suspect that the GPU does not support VP9 decoding. Anyway my Macbook Air 2020 has difficulty running 4k on Chrome, and always drops frame during the first few seconds of full screen on a 4k external monitor when using Safari. Time to buy a new laptop if you want smooth 4k video experience.