User blog comment:B214/Question and Answers 3/@comment-117.195.109.139-20140914155119/@comment-1359322-20140917000456

Well Ishibumi was craftily trying to expand the relationship between the main characters, not just on the love side but from the friendship side too.

And if you analyze properly, you'll see that it's not rushed in fact it's written into utmost perfection.
 * 1) Volume 1-4 was meant as a setup for the series, it introduces the main characters, the setting of the DxD-verse and its main enemy.
 * 2) Volume 5-7 was used to expand and develop the characters, Gasper's growth, Yuuto's change, Koneko and Akeno accepting their bloodlines and Asia finally accepting that she can't be first but will still stay by Issei's side. Volume 5-6 in fact was used to reveal the cornerstone of the series, Great Red and Chichigami.
 * 3) Volume 9-10 is where everything starts officially. With the Breast Power introduced in Volume 6, expanded in Volume 9, Ishibumi made a good timing to make Issei confess to Rias in one of his most important moments in the series, the chance to win his first Rating Game, a fight against an opponent he truly respects, and one of his greatest power-up the Cardinal Crimson Promotion.

And now we've reach till Volume 18, and since Volume 11 (with the exception of 13 & 15) all the Volumes have been full of action plots albeit with a reduction in its ecchiness but still it's been a full momentum since, so 10 (or 9 since 8 is a Short Story compilation) Volumes used to make Issei confess is actually very short plot-wised. It only seem long because the anime only produces a 1-cour season once every few years.