My personal theory would be that listening to full speed Japanese is an immersion activity, while slowing it down and going through line by line is a study activity. Studying is useful in that you can kind of short cut parts of the language acquisition process by kind of giving yourself hints, while immersion itself is the activity that will actually let you acquire the languge.
To take another example, doing Wanikani is a study activity. It teaches you words and gives you a very vague idea of what they mean based on some English word or words. Actually reading and listening to these words in practise is where you actually acquire those words: their meaning goes from a single English word to a more complex standalone idea, you learn to distinguish it from any homophones based on context, and you learn the nuance and tone of the word. If you had never learned the word in Wanikani (or somewhere else), it would be significatntly harder to pick it up from pure immersion, but the immersion is where you really learn the word.
I think listening slowly, looking up words and breaking down grammar falls closer to the Wanikani study end of the spectrum. It will really help you get a good idea in your mind how Japanese use words and grammar to communicate ideas, which you can use as prompts in your mind when actually listening at full speed. But on the other hand you are probably thinking about the grammar and vocab rather than having your “language center” of your brain do it, which I think is far from ideal in terms of actually acquiring the language.
So my conclusion would be to do a bit of intensive listenining (if you allocate 1 hour per day for active study, spend the rest of the time after you’ve finished WK and any other SRS / grammar resources you’re using on it), then spend the rest of your listening time on extensive listening.