Cycles in preview mode isn't bad, you can tune it, and it's also possible to incrementally render in real-time. It looks all dithered, but it works quite well.

Cycles in preview mode isn't bad, you can tune it, and it's also possible to incrementally render in real-time. It looks all dithered, but it works quite well. Its performance will only increase as GPU acceleration options improve. CPU performance is pretty much at a dead-end now, it's barely improved in the last half decade but to slam more and more cores into the chips, while GPU performance is increasing rapidly even now.