User blog comment:B214/Volume 21 info & spoilers/@comment-191.82.149.185-20160704214534/@comment-1359322-20160707033208

Ise doesn't go all 50x [Boost] in all his fights you know. He generally controls it to like 10 or 20x only. That way he can use it more frequently and it reduces his stamina consumption. Remember just randomly boosting it won't help at all, using [Boost] consumes stamina. Using more [Boost] in 1 attack = more stamina used.

Ise used more [Boost] early in the series because he was weaker but he doesn't need to go that far anymore.