There's no way that "resource-intensive and unpredictably timed context switches" account for 100-200ms on modern hardware. With modern multi-core/cpu boxes I think a typical time for a context switch is on the order of 10 microseconds, and that doesn't take into account in things like CPU affinity. (It's still going to depend on a lot on OS and hardware. For example: new i7 vs old core architecture)

If there's actually high costs, it shouldn't be due to a "antiquated" multi-process architecture, but other things like marshalling data to and from the JS engine.