It may not come down to speed. Every jutsu has a weakness and counter. We haven't seen Susaanoo's yet.Itachi said this about JIraya on a purpose. HE was loyal to KOnoha, cant u get that? He didnt want to fight and he didnt want to destroy the viage. PLus, he wsnt ordered to capture Naruto that time. After he got to KOnoha he toyed with KUrenai and Kakashi. He could've killed them both, do you remember? He didnt. Because he was loyal to them.
He, as we could see later, actually loved KOnoha lol.
So, actually, he didnt have a reason to Defeat JIraya.
JUst accept it dude, what speed will Defeat Susano'o if even KIRIN cant. I'm not saying J-man wasnt able to defeat Itachi, we just cant know that.
I agree that it appears Itachi was loyal to Konoha, but it's kind of hard since it's coming from Madara. Though there's a good thread about Madara's conversation with Sasuke and what's true and untrue.