OK, thanks for the additional information. Since it looks the same with the adapter and the built-in HDMI port, then it sounds like an issue with the Samsung. Also, it sounds like you are using it in closed-lid mode?
Because its resolution is only 1980x1080, it may just be there aren't enough pixels, and the MBP is "zooming" the resolution. I would guess that everything is much larger on the Samsung, than on the Retina. Retina display also have the capability of "adjusting" to lower resolutions while maintaining razor sharp images and text, due to the very high pixel density. The Samsung will never come close to the sharpness of the MBP's built-in Retina display. Only a 4K or higher external display would give you a much sharper image. I think it just has to do with the limitations of the Samsung's LCD panel versus the very high resolution internal Retina.
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