Servo has a solution for that: it runs all origins in separate threads. (Note that this goes further than Chrome does; Servo runs cross origin iframes off the main thread too, while Chrome does not.)

Gecko is solving this too, with Electrolysis and Quantum DOM. But because the architectures are so different at the DOM level (direct bindings vs. an XPCOM/direct binding hybrid, tracing vs. reference counting, off main thread layout vs. main thread layout, Rust serialization vs. IPDL, etc.) the Servo solution doesn't directly apply to Gecko. So Gecko and Servo are working on their own solutions mostly independently, rather than sharing code as in most of Quantum.