Good question! For me, the action of actually writing a hub is easy. The hard parts for me are finding the time to write them, creating the data and pictures for the hub and coming up with ideas for new hubs.
Once I have an idea for a hub I research it and think about it until I think it has reached critical mass (in other words I think there is enough to write about to make a good hub). That for me is the time consuming part.
Then I go on to designing the examples and creating the pictures which can also take a long time (in some cases days).
The final part of actually writing takes only a few hours. Editing, proofing and uploading take a few more. It is a labour of love though and a process I enjoy thoroughly.