XP

It doesn't matter what you are writing, there will be days where you'll need 4 hours to write half a page, and days where you can write 7 pages. That's just how writing is, but sadly the difference in quality isn't equivalent to the time spent. During those slower sections, most of that time will be wasted on fact checking everything to make sure everything is factually correct and those efforts will usually go unnoticed. That's why I would personally say that pure fiction works are much more rewarding to write than those based on facts since your imagination can be directly converted into a story. You won't need to waste time making sure that you aren't breaking the laws of physics or be worried that a situation might seem unrealistic. But no matter how much a story is based on fiction, it'll still need a connection to reality and the reader. Thus you will still inevitably need to gather inspiration and sometimes even fact check things.