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Surely if a process is running multiple Isolates simultaneously, they're multi threaded and still require context switching? (accepted, thread switches are less resource intensive than process switches). Interestingly when Chrome runs on Windows desktops it seems to allocate separate processes for each Isolate anyway, but I'm guessing this is not baked into V8?

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Thread switching is much cheaper than process switching.

Threads are just stacks. Processes are whole address spaces. All kinds of caches need to be flushed when you switch processes (especially if you have more processes than you have PCID entries), but not when switching threads.

V8, at its core, is single-threaded. I think that's the reason for separate processes for each isolate.

A V8 isolate is single threaded and you get 1 by default, but other isolates can be created and used in parallel in multiple threads.

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