Haha, that was part of the reason we originally went down this route. In practice, as soon as you want to implement spills, you lose a lot of the benefits because the contents of a spill can depend on any other cell and affect almost any other cell, and you need to evaluate the spill to figure out which cells are relevant. In the end, we rewrote the spreadsheet engine to use a different mechanism that was simpler and that we could optimise better, and then hooked that into SolidJS for everything else.

I guess technically that part of the application didn't scale so well in SolidJS, but it scaled a lot better than it would have in any other framework, and we got to delay writing the engine "properly" until we'd already built everything else and got it running with real users in production.