Hi fork author here :)

I agree, the end game should be to get rid of styled-components. At Sanity we are in fact in the process of moving to https://vanilla-extract.style/

At the same time, with thousands of styled components already in use in production, it'll take a while until they're all refactored. And so it makes sense for us to make the best of styled-components and make it as fast as it can be on the React 18 and 19 baselines, to buy us time for the larger refactor that completely solves it.

TL;DR it's better to never insert CSS at runtime and only link to a cached external stylesheet. If you have brownfield code that inserts CSS at runtime that should be refactored. Until they are refactored, they should insert CSS at runtime in the best possible way (even though it's never truly good or fast).