> At my last company we had a React interface (a specialized IDE, really) that needed to juggle (sort, filter, process) and work with sometimes hundreds of thousands of entities at once, all in-memory.

The author's example inserts 4M elements.

> and the user experience (and the API design) really benefitted from not having a bunch of extra round-trips to the server.

Possibly, but unless you benchmarked both scenarios, this might not be the case.

Performance over networked devices often has trade offs, sometimes those trade offs are directly contradictory, eg sorting/filtering/mapping/etc data on client side versus server side via api call - sometimes one is better than the other but you wont wont know unless you benchmark.

I get the impression that your company did not go through that exercise, considering you guys are building an IDE with scripting language.[1]

[1] https://nickjanetakis.com/blog/switching-to-vscode-from-subl....