When you say slow... does that mean so slow that you shouldn't even bother for a game?
It wouldn't be my first choice. But I have no hard data on perfomance one way or the other. My opinion is that it would be spectacularly slow, but that's no guarante.
You can see a demo of what you can do in the jfc/Java2D demos that come with the JDK.
Where can I learn this in detail?
The 2D Graphics trail in the Java Tutorial is a good place to start:http://java.sun.com/docs/books/tutorial/2d/index.html
The way you say it, it sounds not too hard
Or is it?
It's not so bad. You still have to have a grip on 3D math, so you're not getting away scot free. However, Java2D can handle the intracasies of rendering to the screen.
The primary things you need to understand are:
1. How to convert 3D coordinates to screen coordinates.
2. How to rotate and translate 3D coodinates.
3. How to calculate the distance from a 3D normal.
If you can handle those three, then the rendering should be something of a snap.
One other thing. I said that Java2D could only do Affine texture mapping. (Affine means that a texture is literally cut out of an image, and is thus unable to "tilt" if a polygon is tilted away from the screen.) This is only partially correct. Something called "perspective affine" can be accomplished by pinching and shearing the image in line with the 3D polygon's rotation. However, it's much more difficult to implement, so you probably want to start with simple affine.