User blog comment:Piyush Banerjee/Shin DxD 2 discussion/@comment-2602:306:CD9A:C860:5C15:54F3:D1A2:1CF2-20190216231521/@comment-2601:87:8300:20D0:595F:556D:722C:A1B3-20190217050617

I disagree because unlike you Issei would never underestimate her. In fact she probably never forgive him if she thought he was holding back and Issei knows that. Besides all Rias had to do was stop time or hide in a shadow so it would have been a waste of power and energy to use it that's why Issei didn't use it. It takes a few seconds to charge to fire and those few seconds would be critical. I can't speak for everyone, but if I was in his shoes I would have thought the same thing to be cautious because they know each others abilities.