The game will use between 6.5GB and 10GB of system RAM, depending on what graphics card you have, and the game settings. It could be that. Run something like MSI afterburner, and monitor the Ram/CPU/GPU/vram usage, and see what it's like for yourself. At 1080p assuming your gaming on that res with that GPU (which is best suited to 1080p) then it's likely thats the problem, but you can find out for sure. You said you have tested for overheating etc. How did you do that? To be sure, run an instance of HWmon/Info and take a screenie by alt-tab out of the game. Post it and the Afterburner screenshot and let us know what usage you have.
Also, please list exact specs, mem and mobo, and PSU.
keep in mind it's poorly optimised with many known issues. IMO, PUBG is a dire game. Not talking about the gameplay itself, but rather it's implementation, memory leaks, massive FPS drops, even on decent hardware.