> I really don't see why Safari on a modern iPhone should be so slow at this
me neither, but I don't think it changes the argument. There's always going to be someone on a low-end device. Your adversaries already have superhuman coding ability and infinite patience. Why would you expect the long-term advantage to be with the defenders?
In this case, because there's a vastly more efficient economic path for the adversaries (cloning).
They're not trying to engage in an arms race, they're trying to channel a racing river into its natural course.