changes to global content in lively is slowed down, probably due to mutation observer

[JensLincke]

the rectangles benchmark showed that something is slowing down modifications of html elements in lively compared to running the same benchmark outside.

When running the benchmarks inside a shadow root, the performance gets much better, so we suspect the mutation observer to take its toll.

[JensLincke]

¯_ツ_/¯