I suspect that for someone who reads a decent amount reading speed is also bottlenecked by the rate by which we comprehend the ideas being communicated and not the rate at which we recognize the words. I recently did a bunch of reading speed tests in all the languages I understand. I have like two orders of magnitude more experience in reading German and English than in Dutch, French and Spanish. In the former group word recognition is automatic while in the later I have to concentrate and sometimes even sound out words in my mind yet my actual reading speed for my "strong" languages is only about twice that of my "weak" languages.
Translating is a completely different skill that you have to train on top of being fluent in more than one language. The way I translate is by sort of forgetting something in one language and then remembering it in another. It's a slow and awkward process but I suspect if I did this for like a thousand hours hearing something and then repeating it in another language would be as easy as switching the language in which I'm thinking. I think the real difficulty of simultaneous translation comes from having to speak while you're listening. Consider recording your response to an audio message while listening to it, that would also be very difficult but there is only one language involved.
Yeah. Reading rate varies greatly based on the complexity of the material and how familiar one is with it. It's not just the raw information density, but how much information must be retained and what that entails.
Professionals in a field very often communicate much faster with each other because so much stuff is a standard state plus deviations from that, whereas the person who doesn't know the field can't compress it like that. It's been studied with chess--good chess players are better at memorizing *sensible* board positions, but lose most of that advantage when confronted with nonsense.