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.
Despite the defense of InstallMonetizer, their payment model and practices do not appear to be what you would find with a legitimate software business.
InstallMonetizer has been used by malware as a method to make money as early as April 2011. It was being silently installed by a large botnet, and I assume that the botnet affiliate was making money off the installs.
Their installers are also labeled as a malware by AV vendors, and treated as such by network monitoring infrastructure.
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.