Yes... often JSON serialization and deserialization are the biggest blockers, if they're using puppeteer (as suggested in other comments) that can have its' own instance overhead. I think with ECS they used the "easy" button to start off with and now that they've hit those limits have had to refactor, which isn't unreasonable imho.
I'm working on a project now that's very CPU bound and using limited workers behind an MQ as a distributed RPC behind a fronting API interface so that I can handle scaling... though it's also a Windows-only library involved. There's definitely an art to scaling certain types of workloads and many different options. Sometimes the simplest solution you can come up with really is the best option.
In Javascript world I guess you could do the same by replacing time.sleep with some CPU bound code. eg a big calculation or an infinite for loop.