I usually do around 4Gb of RAM, but I've tried more and less. I just did a bit of toying around with TPPI2 and found no change between Fast/Fancy graphics, Smooth lighting on/off, Render Distance from 10 to 5 chunks and even between fullscreen and the default windowed size (which should hopefully rule out issues with my 1440p resolution being too much). And this is all with FPS unlimited and vsync off, as well as mipmaps off (which did seem to help. apparently that's an AMD gpu thing).
GPU might spike as high as 80-ish% (1 GPU), but typically stays under 20-30% usage and CPU usage is ~1 core's worth. I feel like it should be taking advantage of more, but I've not had any luck in getting it to do so.
Just so you can see I'm not lying or maybe I'm missing something in this:
When I took that screenshot, in game I was in my underground base with only ~30fps. I'm not looking for miracles, I just feel like I should be doing far better than this overall.
**EDIT** another thing to note in that is that the GPU clock never really ramps up. I've tried a ton of different things for that without luck as well. So, even the 80% usage spikes are only at ~40% of the GPU clock, making the overall usage even lower.