To use the Admin in Pagebuilder, the Engine must load up both the Classic and the new Editor. This seems to be a huge performance hit in terms of the time it takes to load Fusion.
This means that developing on Fusion can be time consuming and frustrating, with start-up times taking minutes to complete.
Removing the dependency on the Classic Admin would speed up this process, which would improve the developer experience.
This will be solved in the Q1 2020 release for the new Editor API release