[kbr]

AFAIK React itself is performant enough, the bottleneck is client code within components. I suppose the change is mostly for full enterprise web apps at the Facebook level that likely have thousands of components running at the same time. Concurrent mode lets React split up work and give some time back to browsers (for e.g. mouse movements or rendering) to prevent large freezes on rerenders, essentially allowing prioritization of certain interactions for latency. On top of that, I think it also helps to remove some of the bottleneck that comes from I/O.