I had assumed the slow-down was a result of resource-drain on two fronts: 1) Major updates of the actual Facebook product - I could see a lot of React resource

I had assumed the slow-down was a result of resource-drain on two fronts: 1) Major updates of the actual Facebook product - I could see a lot of React resources being pulled into that. 2) Fiber implementation getting bogged down in the reality of implementing a production-ready cooperative multitasking scheme in the browser environment. I have no evidence for that though - pure assumption.