Though with regards to progress, there's a difference in both approaches.
At the #SolidProject side you have inertia by the slow standardization process. But should they figure things out in a good way, eventually the ecosystem catches up and the inertia can quickly decrease.
While at #ActivityPub side, since AS/AP remains stagnant, the ever increasing protocol decay and tech debt non-linearly increases inertia and progress. And on top of that, you are never done once you implemented the 'ad-hoc specs' of the installed base, and you have to account for continuous whack-a-mole development and maintenance burdens to fix #interoperability breakages.
The AS/AP based fediverse devolves into effectively no interoperability, and a situation that is more comporative to NPM dependency hell.