A fluent speaker can forget how much uncertainty hides inside an ordinary conversation.

Speaking easily removes all evidence of the guessing underneath. Somebody working in a second language is picking up much of what is said and filling the rest from context, tone and hope, and the effort of doing that does not show. From outside, the conversation looks entirely successful to one person and exhausting to the other.

Check understanding by asking what somebody would like repeated rather than whether they followed. Slowing down slightly and avoiding idioms costs you nothing and removes a large part of the work being done on the other side.