Adobe Premiere Pro CC 2017 super laggy playback
What can cause lagging:


Bottlenecks. If you rig is not balanced for a specific task, or has too little firepower in some areas then bottlenecks can occur.
Software issues. Bad drivers, bloated system .etc. Also, premiere isn't the least buggy NLE.


First thing I would check is the system monitor, to see what exactly happens when playback occurs. Does the CPU usage skyrocket to 100%, does the RAM get fully saturated, does the disk get fully saturated? These are all valid questions that will help you find out where exactly the problem lies. If none of these areas are peaking or showing a sign of a struggle, then the problem must lie with the software. If they do show warning signs, then you should consider:


To balance your PC I would add 8gb of RAM and increase the CPU to one of the greater core count i7's (6800k, 6950). Ofc, this will mean changing your motherboard as well as these processors require a different socket type. Premiere doesn't really care how fast your processor is, it cares how many cores/virtual cores your processor has. Case in point, my friend has a new 4 core processor and I have an old LGA2011 (not 2011-2) 3930k; my computer runs Premiere faster and they are both reasonably similar PC's elsewise.
If you are not editing off of your SSD do that now! You need to be able to provide Premiere with a quick means of accessing video clips. It is recommended to have an SSD for OS and apps, an SSD for scratch disks and cache, and an SSD for working projects; only using spinning disk drives for archival or backup storage.


If the software is at fault:


Check to see if this happens in another NLE! Download a trial of Vegas Pro and see if the problem persists there. If it doesn't than somethings wrong with Premiere.
Clean install of drivers for GPU using Nvidia's driver cleaner software, and a clean install of Premiere.


Ideally you want to develop a troubleshooters mindset. You can probably post this quesiton on many forums until you are blue in the face. There are just too many variables that are difficult to diagnose by hunting and pecking at the keyboard. You need to just go through each possible aspect one by one, focusing on the easiest first: (1) work on SSD (2) check other NLE (3) clean install .. etc. Then you can get into addressing whether your computer can handle the footage. Based on my knowledge (which is limited with i5 processors) your computer should be at least able to play 1/2 quality 1080/60 without a hiccup, and 4k in 1/4 quality.

Side note:
I have recently helped someone on a forum with similar specs except a 6700k and a 1080gtx, they were also having issues where the timeline would 'buffer' at the start of each clip on the timeline. Unfortunately we were never really able to narrow it down to any one specific thing. Many other users online have also reported laggy playback on official adobe forums, some of which do not even use newer (and possibly less supported) hardware.



Edit

Also I forgot to mention that the codec you use can make a world of difference. Often people with less than 6 cores find that their computer can't handle figuring out codecs that are compressed using for example XVAC using as their computer is left with the processing:


  In an interframe codec, when a frame is pulled up, the person has to look at the frame before it, and after it and then decide how to unwrap it – before unwrapping it.


In this case, and for troubleshooting, it might be a good idea to convert to DNXHD an intraframe codec.