Every expert carries memories of not knowing what now feels obvious.

Everyone who now finds something obvious once found it opaque. The expert has simply forgotten the shape of the confusion, which is why explanations from very skilled people so often skip the exact step where a beginner gets lost. The knowledge is real and the memory of not having it has faded.

If you are learning, take slow explanations as a sign of a good teacher rather than a slow subject. If you are the one explaining, try to recall your own confusion, or ask the learner where exactly they stopped following. That question recovers what your own fluency erased.