After an earthquake damaged Amagi, Japan chose Kaga as the replacement conversion. Carrier operations already rewarded speed and airflow over the flight deck, while Kaga's battleship hull was comparatively slow. Japan could instead have asked the treaty powers to permit a purpose-built carrier, even at lower displacement. A new hull might have used its allotted tonnage more efficiently than a conversion. This was a possible design and negotiation mistake, although Japan had no easy substitute ready.
Notable Point · 47:46-49:09
Why Kaga was a poor replacement conversion
After Amagi was damaged, Japan could have sought permission for a purpose-built carrier rather than accepting Kaga's slower battleship hull.