(Redirected) Dell 4k P2815Q refresh rate issues

Hi,

I bought the P2815Q 4k monitor. I'm aware it runs on only 30Hz refresh rate at 2160p - which I thought would be good for watching videos (which I tested it is).

But when I put it to 1440p or lower I can still feel it's running on 30Hz even though I change the settings to 60Hz - no difference at all and it feels laggy. I want to run it on 60Hz on lower resolution.

I'm using Display port Mini - is that the cause maybe?

My desktop is high performance so that's not the cause, with Asus R9 270x GPU which I doubt is the cause.

How can I fix this?

Thanks!