What is the best way to connect an external 4K display to an iMac 2015 for optimal performance I'm experiencing some kind of ghosting problem when connecting my iiyama 40'' monitor to my iMac 27'' retina 5K display (osx 10.11).  Not sure about the term ghosting but when moving screens (and mouse pointer) on the external monitor, they behave sluggish and the contrast whith the movement on my retina display is huge and therefor a bit annoying (it is not display lag).  I also had the same problem when connecting my iMac to a Samsung 4K tv (UE40JU6000) even in 'game mode'.  When connecting my older Macbook pro (late 2009) to the samsung tv or the iiyama the 'ghosting' is gone and the screens and mouse pointer are moving very smooth, just like on my iMac. 

I made the connection with a Belkin 4K mini-displayport to HDMI cable both for iMac and Macbook. For the iMac I also tried a displayport cable (deliverd by iiyama) without improvement (not sure if its 1.0 or 1.2 displayport)  

I also tried different resolutions on both screens and all available HDMI-ports on my iiyama.

The HDMI 2-port on the iiyama looks like the one to go. This port is suitable for 3840 x 2160 at 30 Hz.  

Apple stating 30 Hz connection:
https://support.apple.com/en-us/HT206587

full specs iiyama: 
http://www.iiyama.com/gl_en/products/prolite-x4071uhsu-b1/

Any help would be highly appreciated

Rob
Some displays allow you to specify the DisplayPort version. If your display defaults to DisplayPort 1.1, changing it to DisplayPort 1.2 via the OSD might improve the DisplayPort connection.