1. We saw how itachi Countered Nagato abilities which are stronger and Nagato is also way faster at doing them according to Naruto. Yet Itachi amaterasu worked + yes, you aren't considering the speed gap here, Itachi can potentially stop Deva to even start Chibaku. He scales to Naruto KCM body flicker in speed. Pain bodies are slow as compared to. That.
2. Road to ninja who was supervisionated from Kishimoto had Obito get away from Itachi, scared. The novel as well, had Itachi being initially below Obito and scared of him and then ready and confident on taking him on later in the story when Itachi got stronger.