First, not sure what your resolution is at, if it is on 4k or 1440p, then it is too high.
Second, PUBG is horribly optimized right now. I run twin 1080 ti's in sli and I still have frame drops, although on max settings at 4k. But given the games popularity, I have every expectation that the game will gain better optimization as it is still in early access and not complete.
Third, your processor and RAM is very close to the minimum requirements. The minimum requirements for CPU and RAM are i3-4340 and 6gb. Your CPU may be holding you back. I have not seen benchmarks with the game with various CPUs, but it is a CPU intense game considering there are 100 other players on the same map as you at a given time.