Writing is about solving problems of expression.
The difficulty of the problem depends upon the difficulty of the idea you want to express. Some thoughts are easy to put into words; the ideas you know well, have confidence in and have explained before. Other ideas take much more work, maybe because the thought isn’t fully clear to you, or because it is a subtle point or requires deep insight. These take much more thought, effort and time.
If you only write fast, then you will only be able to write about the things that come easily to mind, and you will never be able to reach those deeper insights, nor be able to express those difficult concepts adequately. It you only write fast, then you are denying yourself the opportunity to solve those more difficult (and often important) problems of expression.
A lot of writers get frustrated if they can’t maintain the same pace, but in order to write well – and to cover the full range of difficulty – you must allow your pace to vary with the difficulty of the ideas you are trying to express at any given time.