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So, first things first. Your ram is running at default speed. You need to go to the bios and set the ram onto XMP profile. Run CPUZ and open the SPD tab. Select a memory module and note down the timings and XMP profiles. Go to the bios, set XMP on, and select the right speed. That could very well be the cause of your issue.
Also, don't get too hung up on a 'bottleneck'. Your CPU is very capable, and although in very few games you may have a very slight bottleneck (because the max speed of your CPU is 3.6ghz). If there is a slight bottleneck (no more than 5% IMO) then you can offset that by OC'ing your CPU.
In the meantime, run HWMon, and let your PC idle. Take a screenshot of HWMon and post it here. Then run BF 1 and alt tab out of the game (while HWMon is running in the back round) and then take another screenshot of HWMon. This will show us whats happening with your hardware while you have the issue.
Get started with that, and let us know.
Also, the work at 4k is all done by the GPU, the CPU is important, but less so.
Ans lastly the biggest question! What is it that makes you think your performance is not up to scratch. Cores maxing out or not is not that relevant. If you weren't getting the FPS you thought you might, that would be a better question.