Thought I'd put this here before editing in because this is not absolutely confirmed. At least on my machine I needed to get an SSD to stop very bad stuttering. JC3 reads in a LOT of data because the world has a lot in it and I've done tests to show that this is the case - http://blog.djbarney.org/just-cause-3-on-the-pc-is-not-badly-optimised-it-has-high-storage-medium-speed-requirements-with-evidence/ ... At least to me it seems that the "badly optimised" criticism, although understandable, is actually due to badly published requirements for the game. JC3 either requires a lot more memory than published and/or an SSD fast enough to prevent the engine waiting for data (stuttering).
Topic on Talk:Just Cause 3
How much RAM do you have? Also considering how this game looks like I honestly don't feel like it should be running this poorly, on my system I should be able to run JC3 at over 100+ FPS. The Witcher 3 runs and looks better than JC3 for the most part. The video settings in JC3 barely help, and the engine really struggles when there are a ton of civilian cars on screen in certain areas of the game.