When comparing octane-regexp on linux/windows it is visible that readBarrier is taking more time on windows. It takes 54ms of the time, while on linux only 18ms. I'm esp. talking about: js::InternalGCMethods<class js::SavedFrame *>::readBarrier (which in linux has js::InternalGCMethods<class js::JSObject *>::readBarrier as signature) I'm still not entirely sure what the difference exactly is though. But on both platforms shadowZone->needsIncrementalBarrier() JS::GCThingIsMarkedGray(thing) is false. So we this is actually a nop.