MacBook Pro (Mid 2014) 4k external displays performance
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.