My PCIe Bandwidth theory actually worked? I mean... I totally knew that was it!
It makes sense but what about everyone else with Lagged frames? Do they all have low PCIe bandwidth? There's not enough people with issues to determine this.
I opened PUBG and GW2(Both 1440p max settings) with OBS(720p60) all running in Borderless, 99% GPU, no lagged frames other than from tabbing around.
People open PUBG and immediately get lagged frames, OBS FPS counter drops. The default answer is to "cap FPS" to reduce GPU usage, but that is not a solution in my opinion. Lowering your GPU usage masks the underlying issue, it's not that clear cut, it's not as simple as GPU "usage".