Congrats on the launch, this looks awesome! Would love to learn more about how it was built.
Given there are definite limitations with browser based JS and user’s systems, we’re now a fully cloud based compute environment. As users make changes in our React UI, we send the work to a queue and then distribute it by parsing the flow tree across various node worker servers so steps can run in parallel (if your flow is built that way). Each step is self contained and we then use sockets (powered by PubNub) to notify the UI of changes.
AWS Lambda is fairly reliable for running untrusted functions (as well as trusted functions with carefully managed inputs) at scale for customers, but any function provider should work well for this purpose (as long as their container lifecycle meets your needs).