How slow the shadow creeps: but when ’tis past
How fast the shadows fall. How fast! How fast!

Belloc’s lines juxtapose two felt experiences of time: the slow, almost imperceptible movement of a shadow while it is happening, and the sudden realization—once the moment has passed—of how quickly darkness (or loss, age, death) seems to arrive. The repetition (“How fast! How fast!”) mimics the startled, breathless recognition that retrospection compresses time. The image of shadows suggests both literal evening and the metaphorical “shadows” that gather over a life, implying that we often underestimate change in the present but experience it as swift and irreversible in memory.