Belatedly is the adverb: in a belated way, later than it should have been. It often sits with verbs of noticing, acting, or saying sorry: they belatedly realised the mistake; the government belatedly offered help. It sounds a little formal, the kind of word you meet in news and careful writing more than in quick chat, where people might just say too late or finally. It does not mean “slowly”. It means the timing is off — the moment has passed, and now this happens anyway. You can feel a small sigh in it, though actually it can also be gentle, as when someone belatedly remembers your birthday and still wants to be kind.