Could you please post a screenshot of your game while you have the F3 debug screen enabled?
Also, as a side note, never allocate more than half of your system's RAM to one process. More RAM does not equal more FPS and can actually hurt performance if you allocate too much. Since you have 8GB of memory in your PC, 4GB should be the maximum that you allocate to Minecraft (and you shouldn't ever need that much unless you are using a lot of mods).
From the screenshot it seems that the system is using the R9.
I see some artifacts on the farthest torch, so probably you have anisotropic filtering and antialias enabled by your video card. See if there's a panel where you can disable them globally or for minecraft only.
I was gonna mention that earlier. You have a pretty decent GPU but not a very powerful processor, which could be causing a bottleneck. Does your computer have any performance issues running other modern-ish games? Unfortunately, the only way to fix a CPU bottleneck (if that is, in fact, what's causing the problem) would be to buy another CPU and replace the one you already have.