At its core, Java3D uses either OpenGL (or DirectX on Windows systems), so you won't escape the slow rendering, no matter what. As for software rendering being slow, well...yes and no.

Software rendering will always be slower than "hardware" rendering because you don't have a dedicated chip doing the work. But back in the OLD days (97 and before), we didn't have hardware TnL. It was all done in software, and it all ran pretty fast. The basic transform and lighting algorithms are not computationally expensive for small numbers of lights (2-4) and polygons (1-10,000). The most computationally expensive part of 3D graphics is alpha-blending, per-pixel lighting, and basically everything the shaders today save us from. :)

In fact, if you're interested in doing something within Java, consider looking at Developing Games In Java, by David Brackeen. Or at least the posted source code online. In the book, he creates a software 3D renderer in Java and using a scripting engine that runs about 30-40 FPS. It's really quite eye-opening.

But, if you want to make use of newer technologies, you'll have to use something that's capable. OpenGL and DirectX both have software versions, and run fairly quickly. Thus, Java3D will also run quickly. There's probably something similar to Java3D with C# as well, but I have no idea about that.

Even a 'low end' computer bought in the last 10 years will be able to run graphics at a reasonable speed, as long as you stay away from 'cool' stuff like shaders and real-time shadows. Think about Quake II (1997) and how nice its graphics can be. Just allow the graphics to be turned down a long way and you should be fine.