Instant is not the only speed grace comes at, though it is the one that gets the stories told about it. A candle lit in a hospital chapel at three in the morning rarely produces a thunderclap. What it often produces is a person who can walk back to the ward.
Whatever you call the thing that flows through — spirit, grace, the plain fact of being held by something larger than the situation — it seems to work at both speeds. Sudden when it is sudden. Patient the rest of the time, which is most of the time.