I am a second year PhD student working in math.
I find that in most discussions with postdocs or even my own supervisor, the pace of the discussion is too fast. The other party invariably seems to think faster and go through to the next step before I have fully processed the current step. I usually have to take photos of the whiteboard, go back, work it through and then realize either that the argument is correct or that there was an error. At conferences or talks, the same is true - the pace of the discussion is too fast for me to meaningfully contribute. I mostly end up listening and this makes it difficult for me to find collaborations because the conversation is too one-sided. If there is no whiteboard and the discussion is over a meal, or when there are multiple people involved, it is that much harder!
While some of it is down to experience, I imagine that even experienced researchers working in fields that are new to them face this issue. How does one "informally" brainstorm with others in an effective manner in technical fields?