Optimising geometry

This page needs vetting by someone who understands rendering. Believe it at your peril.

• There isn't an established polygon budget, but aiming for less than 10,000 at highest LOD seems reasonable.
• More important is to use LODs well.
• All faces are triangles as far as the engine is concerned. A more complex face is reduced to triangles automatically - there is no need or discernible speed advantage to doing it yourself in the modeller.
• Vertices and edges don't matter as such. If you create three vertices, create edges between them, divide those edges into parts then create a face between the original three vertices you still have a triangle. In Blender, when you select a face the delineating vertices will be highlighted - if there are three of them, it's a triangle :-)
• Try to keep your geometry in quads. This means squares, when importing the engine should automatically triangulate the mesh.
• Avoid Ngons, these are faces with more than four sides. An Ngon will cause shading issues and a whole bunch of problems so try your best to avoid them. 
• Removing double vertices often helps to get rid of extra faces - under Mesh Tools in Blender
• Backfaces and internal faces are bad, because they waste renderer cycles and risk creating glitches where a face shows over a face it should be behind. If you're building your model using geometric primitives, be sure to remove all the internal faces where possible. In Blender 'local view' or use of layers help immensely. Set your objects to single sided unless you have specific need to use faces with two sides.
• That said, there is a balance between removing internal faces and overcomplicating the geometry. Too many triangles strains the renderer. If you've got a small mesh intersecting a face, cutting the face up to meet the mesh introduces extra triangles, and is a judgement call.