MacBook Pro (Mid 2014) 4k external displays performance I was using my MacBook pro for years with two external full HD displays without a problem. Recently I switched to two 4K displays and since then fans in my MacBook spin all the time when when hooked up to external screens. I see that WindowServer spins at 20-30% CPU most of the time even if I do some basic activities.

Is this something expected or is it fixable and how?

I'm on High Sierra 10.13.6 with 16 GB ram, i7 and GeForce 750M with 2GB ram.

When I tried to watch some 4K youtube video I experienced stutter and overall poor playback quality. The same video in Full HD runs smooth.

[Edit]

I tried the same on Mid 2015 MacBook Pro equipped only with Intel Iris and it has no problem supporting 2 external 4k screens. Does one year in those models makes such a big difference? Or there is something wrong with mine?
Assuming you went from 1080p to 2160p your total screen area has gone from 4 megapixels to 16 megapixels.
If you actually mean 'full 4k' 4096 × 2560, then your screen area is now in the order of 20 megapixels.
That's aside from the internal screen.

You're just expecting too much of it, to be able to push that kind of resolution at full speed.

Spec for the 2014 vs 2015 - source Everymac


2014 - 2 external displays over Thunderbolt 2 @ 2560x1600
or a single display up to 2560x1600 via Thunderbolt and a single 1080p display at up to 60 Hz, 3840x2160 at 30 Hz, or 4096x2160 at 24 Hz via HDMI.
2015 - 2 external displays over Thunderbolt 2 @ 3840x2160
or a single display up to 3840x2160 via Thunderbolt 2 and a single 1080p display at up to 60 Hz, 3840x2160 at 30 Hz, or 4096x2160 at 24 Hz via HDMI.


So yes, it would appear the 2015 has significantly more grunt for video, even though it geekbenches about the same as the 2014 overall.