What would you rather have, in this case? Assuming that the work needs to be done? You can either do that on allocation (which you can predict, batch, etc) or

What would you rather have, in this case? Assuming that the work needs to be done? You can either do that on allocation (which you can predict, batch, etc) or you can do that when you need the next page to be used. Which is far harder to predict and work with.