That's a good question. It made me think again about why it takes me so long to write a hub. Out of the 80+ hubs I've written, a scant handful have taken less than a day, and most have taken many days to write.
Aside from content, I probably spend the most time on layout, meaning choosing and arranging the types of capsules I want to use to convey different kinds of information. I like to keep a hub's page visually interesting, so I use a lot of half-width text boxes, sometimes tables, and always photos or videos or both.
Another layout aspect that takes up my time is previewing a hub in progress to see how HP and AdSense ads are going to appear on the page. To avoid annoying empty white space caused by those automatic ad placements, I sometimes have to break down one text block into several capsules so that the ads "fit" the layout instead of control it.
My goal (aside from writing accurate and engaging text) is to create a visual design that lets the reader move easily from one idea to another, because there's almost no better way to lose an online reader than to make him plod through dense text or navigate meaningless white space in order to understand a message.