I'm guessing with your current situation, you don't have a great graphics card.
For most game related issues on fps lag, it is mostly due to the graphics card, or sometimes the CPU. But seeing as how the Mac is very limited with the choices of graphics card, this seems to be the most logical reason.
You will need a better graphics card, which the Mac can't give you. (Or if you plan to dish out $2000 for a 680m Mac) My suggestion would be get a PC with an NVidia graphics cards 460GTX, 560GTX+, 660GTX +, 760GTX+ or AMDs 6700+, 7700+, R92xx.
EDIT: John is partially correct, lowering your settings will increase FPS, but internet connection has no relevance.
Now looking at your specs, your CPU isn't terrible, but it isn't great. As for the RAM, recommended amount is 8GB, but 4GB is still fine.
However as I said, it seems like the graphics card is the issue, actually I'm 100% sure it's the graphics card. As it is in the name itself, "GRAPHICS" card, it controls how smoothly games can run, as long as the CPU can keep up. The more expensive graphics cards obviously provide better performance, and there are hundreds of different graphics cards, while the Mac is limited to a small fraction of those graphics cards, most which are lower tier. Thus providing horrible FPS with games. That is why I am saying, your only solution is to get a PC, with a decent graphics card and CPU.
EDITEDIT:
Well, you'll have to be more specific with the specs. do you know what the graphics card exactly is? Also, the hard drives won't make a much of a difference in gaming performance. The only result you'll probably see with a better hard drive is in loading time. Another issue could be heating, as overheating can cause tremendous lag.
EDITEDITEDIT:
That's an old graphics card, but it should still be able to handle Rust. My apologies for the rude attitude on graphics card (speculated you had a crap graphics card). It might be with overheating or programs in the background hoarding memory and CPU usage. Try turning off some programs in the background and check the temperature of your CPU and GPU.