That's probably because believe it or not a Core i5 (any type) is not sufficient enough to smoothly run the game. It completely murdered my i5-6600k with a 5.1GHz OC and a GTX 1080ti. Switched to Ryzen and poof! Problem went away.
This game loveees logical threads, something the i5 doesn't have many of. Not being biased, but i've done extensive research on this issue and that's the conclusion i came to. Even my Ryzen R5 1600 is pretty taxed at around 50-57% with all 6 cores/12 threads being used.
The age of "oh well the i5 is the best gaming CPU" are over, more and more games including this one are migrating over to using more core/threads instead of relying on CPU IPC/Clock speeds which is a limiting factor when all they have to optimize on is 4 cores and 4 logical threads (8 threads is still good for now, but 4 is trash for this game).
If you want your i5 to run decently, crank the settings up all the way and enable the launch option to allow more RAM to be used since the CPU caches on a i5 are pretty small and it only has one scheduler per core. You're going to want to max your GPU out, when you do that run the launch options that force the game to use all available CPU resources as well and set the .exe priority to high. Since then although your GPU will be maxed, the CPU will get smacked with a load also, but will help buffer out some of the FPS spikes you get from being CPU bottlenecked by having the GPU take most of the bashing.
One last thing, set a page file and if you have another drive, do it on that. Even if you have 16+GB of RAM, for some reason this game likes to page (my performance counters for page faults skyrockets to 21,000,000 when PUBG play's without paging enabled).
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