Why does my reading comprehension drop when I move from subtitles to actual Icelandic TV?

I’ve been watching 'Ófærð' (Trapped) with Icelandic subtitles, but as soon as I turn them off, I feel like the language speed quadruples. It’s not just the vocabulary; it’s the way the declensions seem to blend together in spoken natural speech. For those who have successfully moved to native content, how did you bridge the gap between reading a word and actually hearing it in a fast-paced sentence?