As someone who doesn't pay, your only hope is to get someone as excited about the project as you are. Projects can take years to finish and even longer to make any money. A good graphic artist can probably get paid today and still work interesting projects.
The reason your guys are slow is that this is probably a casual side project for them. It doesn't mean they have no interest. Just that their interest or commitment level is currently less than yours.
That said, I have paid good money for contract art and sound and still not gotten timely or satifsfactory work, so there is definitely a shopping around period until you find someone you are reasonably comfortable.with. Also, you quickly learn different artists have different fortes. Some are great at characters, some brilliant at machinery, some are great interface people. And some are a bit slow, but worth waiting for.
Also, just consider paying something. Seriously. Do you have a day job? Set some money aside. If you don't have one, be a contractor for a bit to get some cash. Do you honestly expect your project to make money? Then that's one less mouth to feed when it does. You'll be happy they're already paid off.