If you've got a limited attention span, jettison all backstory, character development, plot, etc. Just get straight to hardcore explicit fucking, focusing on nothing but disembodied genitals and BDSM madlibs. I suspect that for many readers, that's all they want. And for some authors, it may be all they're capable of. You can churn these stories out to your heart's content with little effort required. It's a great way to get impressive publication stats if that's what drives you, but it won't really differentiate you from the literally thousands of other writers writing these kinds of cookie-cutter porn-scenes.
However, there are some readers who actually look for quality storytelling in their stories. To do that well, establishing backstory is essential. It's especially important for the author to understand their own character's histories and motivations. Who are they? Why are they here? What is the nature of their relationship? What drives them towards each other? How do you explain the ways they interact with each other? What challenges do they have to overcome to get to what they want? How much of that makes it onto the page may be another matter. As Ensorceled and Seeker mentioned, backstory can be revealed through dialog and action. Too much explication can bog things down. But backstory can't be an afterthought or completely neglected because you (as author or reader) are too impatient to get to the end. My focus is more on building momentum throughout the story leading up to the big payoff (sex). I hate being thrown into the middle of an anonymous sex scene with characters I know and care very little about. I need that back story in order to be engaged. At the same time, the back story has to be relevant. If you set your story on a cruise ship, I want to know what these people are doing on the cruise, etc., but I don't need the entire history of the American shipbuilding industry, nor the characters' reminiscences about their childhood pets unless it has some bearing on how they're perceiving the present events of the story.
From a writing point of view, I think the best process is to write whatever's on your mind in the first draft, fill out what needs further explanation or clarification in the second draft, and then mercilessly trim anything that doesn't actively deepen our understanding of the characters or move the plot forward in the third draft. Then spell/grammar check and publish.