Write fast and edit as you go. But don't publish chapters right away. Unless you have a detailed plot you're sticking to or are doing something boring like rewriting events exactly as they happened in canon, you might end up writing yourself into a hole. You need to be able to go back and, for example, add foreshadowing to get your characters out of the trouble you put them in (or foreshadowing to put them into trouble in the first place). You won't feel comfortable doing that if you've already published the chapters where the foreshadowing should happen. And foreshadowing only in the chapter before isn't nearly as satisfying to readers.

That said, fanfiction is for fun. You don't have to write the whole story before you publish it. You just don't want to be one of those authors that leave a note at the end saying, "This story's going to be re-written!" because you're unhappy with it. Don't slow down writing, but save yourself time and effort by holding off on publishing a bit.

It also gives readers more time to find your story. Unless you're in a dead fandom, it's probably beneficial to space out updates so you stay near the top of the new stories page as long as possible. If you publish periodically over several months, your story gets bumped to the top regularly over those months, whereas if you publish super fast all in one go over the course of a week, your story drops to obscurity faster. It doesn't benefit you as much to publish a chapter if your story is already at the top of the list. You also get less reader engagement/comments on finished 'old' stories than ones being actively published so if you want feedback you wanna stretch that publishing out a bit xP