Nvidia locked down a lot of functionality on their cards to start with. OpenCl runs slow as death on Nvidia cards and Cuda is better but still isnt great. Their cards have potential but with how they run now they will never be good for hashing. Nvidia designed their cards with graphics in mind not computing, AMD did both.
Your 680 is on par with 7970s/R9 280x and most of us run 3+ of them. The main reason why people don’t use nvidia is power/KHs ratio…its the same reason we don’t use CPUs to mine any more. I liked my Nvidia cards more than my 7970s when I first got them but AMD has put a ton of work into drivers and that has made a huge difference.