Serious Graphics Question

First off, let me start with PC Specs

AMD A10 Quad Core
ATI R9 390
6g Ram
Samsung SSD (250g)

Does anyone know why, when I run the Game in Fastest/Fantastic, I get roughly 40-60fps. But, if I run the game on any setting in between, my FPS drops below 30, and most of the time becomes unplayable? Playing on Fantastic is fine, but usually need to do a restart every few hours as the FPS slowly degrades over time. Playing on Fastest, I’m able to play consistently w/o any FPS drop at all. BUT, when I go for one of the choices in the middle, FPS is all over the place.

What settings change between Fastest/Fast? What are the differences between Fastest/Fantastic? None of the graphic settings change as far as I can see, so wondering what settings behind the scene change between all these levels. Is there a place I am able to find these settings? Honestly, good graphics or not doesn’t bother me at all, and I don’t mind running the game on Fastest, aside the fact that Rocks/Ground are very warped until you are right on top of them, making PvP hard sometimes as people can be right in the open on Fantastic Setting, but Fastest, a rock would be blocking them. (this happened a few times tonight when playing with a friend. He could see someone, all I’d see is this horribly misshapen rock that was hiding them until they moved more to the side into the open.