Tuuj, the only characters to ever show exemption to fatigue to carrying a large weapon were Zabuza and Kisame, one of which could literally fuse with his sword. Suigetsu even grew exhausted from carrying the Executioner's Blade. Although at the moment I don't see a problem, Shira will have to show IC exhaustion from using the blade. She's also quite young - I know grown men that can't hold a Halberd correctly (funny story, but I digress). World of Naruto gets to bend those rules, but don't be too unrealistic. She will grow exhausted very quickly during combat and there is no way I'm going to allow her to be a swift sword master at this level. Her slashes will be crude, her style imbalanced and her follow-through uncontrolled. That sword literally has nearly ten times as much as a katana, assuming it weighs the standard 1.5-2lb. I know personally that 15lb weight vest alone can make menial exercise exhausting.