Berenice,
I honestly think that you'll run into a lot of work if you try to create and use your own driver. I'd recommend that you pursue another route.
You mention wanting to use real-time for "fast, not exactly predictable" performance. Real-time is often misunderstood. It does not mean "fast." But it DOES mean "predictable." That's the reason you choose to go with a real-time system...for the determinism that it offers. If you don't care about being exactly predictable, but do need something fast, I don't even think you'd need to go the real-time route. I'd recommend a Windows system in this situation.
If you still want to pursue the real-time route, you'll need some sort of device that can run a real-time operating system, whether that be another PC or, possibly, a PXI chassis with a real-time controller. You can find plenty of information on our website about setting up real-time systems. http://www.ni.com/realtime/. In the event that you choose real-time as your direction, you'll need to make sure that you have the LabVIEW Real-Time Module. If you don't currently have it, you'll have to look into purchasing it. That module will allow you to target the real-time machine and run your developed code in that environment. I hope that is helpful to you. If you would like more information on setting up a real-time system, I'd recommend that you call and speak with one of our technical representatives at (888) 280-7645.
Jim Laudie
Applications Engineer, National Instruments