User blog comment:B214/Volume 22 spoilers/@comment-154.242.38.77-20160819230516/@comment-95.90.227.125-20160820122744

You must see it the other Way. Vali trains for Years and not one or two but like 7 or 8 Years when Issei is only in the Supernatrual for a little under a Year. So Issei`s growth  is much more awesome than Valis. I think at the End of the IRG Issei will be Ultimate Class. And like it said in Vol 21: "Rias Gremory’s servant ‘Hyoudou Issei’ was nominated for a promotion to become a high-class Devil. Being advocated for a promotion to a high-class Devil within less than a year after reincarnation, this was an exception among exceptions that had never occurred in the past.