A nanosecond is a very short period of time, so the fact that the simile uses it to describe how quickly the feeling disappears suggests that it vanished almost immediately.
Consider the statement "I liked my brother's girlfriend for a nanosecond, but then I remembered how she controls his life, and that feeling went away fast.". Is the statement "For a nanosecond I like my brother's girlfriend, but then I remember how she controls his life, and that warm and fuzzy feeling disappears like a streak of lightning." false? Yes/no/unknown