This release contains a lot of memory allocation optimizations, which should also lead to better performance as there is much less garbage collection going on than before. Also the codebase has now better structure as it's split into logical modules. We are now using Rollup as the build tool and Prettier as the code formatter.
The public API is pretty much untouched, but internally there's been a colossal refactoring process. So if you relied on any of the internal APIs please make sure to revise those parts before upgrading to this version from an older version.
Here's a high-level overview of what's changed: https://github.com/haltu/muuri/milestone/6?closed=1