There are many, many factors that impact performance. I've categorized them in a way that should allow you to experiment with different options. The best performance is often achieved by multiple small optimizations, and there is often a tradeoff between quality and speed (see quality). Remember, if you improve 10 small things, the overall impact can be dramatic. It is often possible to speed things up 100% or more.

Things in the environment that affect performance but may be beyond the developer's control include:

• Flash Player version
• Browser version - Firefox, IE, Safari, etc.
• Computer's memory, graphics card, CPU speed
• Internet connection speed
• Other applications running simultaneously
• Operating system - Mac, Windows, Linux - various flavors
• Whether it is running locally or remotely.
• Whether it is running in authoring mode (Flash's debug mode is extremely slow), a browser, projector, or third-party EXE such as SWF Studio.

If your application is running slowly, test it on another machine. In the end, you may need to scale back your application or specify a beefier minimum playback platform. Or you might require a later version of Flash Player.

Things that affect the rendering speed of individual models:

• Skin, skin, skin! - The skin type can have a dramatic effect on performance. Try using a SimpleColorSkin or SimpleLineSkin for speed comparisons.
• Shading - turn off shading with Skin.setLightingEnable(false) in Sandy 1.1 (I'm not sure if that is the same in 1.2)
• Alpha/transparency - set alpha to 100, which renders fastest.
• Flash 8 vs Flash 7 - publish your project in Flash 8, which has some faster rendering options - I haven't tested BitmapScreen mode in 1.1 (and I think it is buggy) but using Flash 8's bitmap optimization should be faster.
• Number of polygons or faces for an object. A sphere renders much more slowly than a pyramid. Try reducing the quality setting, such as for a cylinder, to reduce the number of polygons.
• Avoid complex models, such as those imported from ASE files; reduce/simplify the model if possible.
• Backface culling - turn on backface culling for better performance (set Object3D.enableBackFaceCulling = true). See also Object3D.swapCulling();
• Use smaller images for your TextureSkins.
• Turn off animation of MovieSkins.
• Avoid VideoSkins.
• Does turning on/off Light3D affect performance? Probably.

Things that affect the overall performance of the world:

• The rendering engine. Are you using Sandy 1.1? Maybe some of the optimizations in Sandy 1.2 will help.
• Consider reducing the number of objects in the world. Add objects one at a time to see when performance degrades. See also section above regarding simplifying individual objects.
• There are ways to combine transforms for better performance. For example, instead of nesting three transforms, you can calculate one matrix manually.
• Consider reducing the depth of the object hierarchy. For example, if some objects don't move or rotate independently, they don't have to exist within their own transform group.
• The size of the world and how fast things are moving affect the redraw area. Test it with a smaller world or less movement.
• Increase the frame rate to increase the number of renderEvents being sent per second. Keep your render handler short/fast.
• Reduce ther buttons and elements in Flash outside your 3D Sandy world. Don't forget to minimize the “chrome” around your viewport.
• Loading external content can be slow, especially if it is from a different server or large media.
• Reduce other heavy/complex ActionScript code or eliminate poor programming techniques.
• If things slow down over time, check for memory leaks. Be sure to dispose of things you don't need any more.
• Loading images dynamically at runtime can quickly consume a lot of memory - use smaller textures.
• At authoring time, avoid a lot of trace() statements or updating text fields dynamically as these consume performance. Update them as infrequently as possible (i.e. only when something changes rather than constantly).
• Use an optimizing compiler such as MTASC but beware as it might introduce other problems (test carefully).
• Employ generic Flash optimization techniques as above.