The v5 release experienced a great deal of internal refactoring to improve modularity and performance but also resulted in a 20% reduction in size compared to v4. The API however stayed relatively uniform so migration shouldn’t be difficult, with a supporting migration guide available here. The new version includes new sprite batching support that should have a pretty profound effect on drawing several different sprites. The Graphics object also received a few new tricks such as now being cloneable, supporting holes, texture fills and more.
Perhaps the biggest new feature is the new mid level API that has been added, that Pixi itself builds on top of. Describe in the announcement blog like such:
Ok so I saved the best till last as I think this isn’t far the coolest part of v5. The midlevel API. We created an API that basically abstracts as much of WebGL as possible but still keeping all the power, giving you guys the power to create awesome WebGL that is automatically optimised for you!
In fact all of v5 is built on top of this API. Expect a more detailed post and examples for this one soon.
Additionally, the community created an excellent new tool in the form of the browser based Pixi Playground, a complete editor and runtime environment for playing with Pixi applications.