The issue is writing style. Sometimes people use a slow, careful writing style, agonize over word choices, and edit repeatedly. This extra time spent on writing doesn’t actually do a good job of catching and fixing topical errors. It’s a different activity than doing more thinking or research. It can actually be misleading: polished writing gives the impression of knowing what you’re talking about, having fully-baked ideas, doing appropriate research, etc.
For thinking and research, feel free to spend time on those.
FYI there is an issue where sometimes people don’t get external feedback early enough, but that’s on a different timescale than one writing session. Like if you spent the next month researching something without posting anything about it, and it turned out to be something someone here already knew important criticism of that they could have linked if you had just mentioned it, then that’s inefficient. Similarly, I’ve seen lots of people finish an entire philosophical book before they mention it, and they had some misunderstanding near the beginning which affected their interpretations of most of the rest of the book. So there’s a risk involved with late feedback there. But this is a separate issue than how long you spend in a writing session trying to optimize your wording.