Improve educational materials around performance

A lot of people on the forum end up asking about how to improve performance for some specific use case. It would be nice to have a central resource discussing some common issues (eg. entities vs primitives)

Here are some helpful threads:
https://groups.google.com/forum/?hl=en#!searchin/cesium-dev/primitive$20custom$20property%7Csort:relevance/cesium-dev/ySKnWXFydgY/1PgFb6xvXYIJ

https://groups.google.com/forum/#!topic/cesium-dev/QJdWFSGcJKo

Start by reading the two linked Cesium forum threads and identify the recurring performance questions they illustrate. Define the scope and structure of a central educational resource, including common topics such as entities versus primitives; done means the resource is published and addresses the selected use cases clearly.

Written by the indexing model from the issue text.