Why the IJN called the Kongos fast battleships

Why did the Imperial Japanese Navy consider the refitted Kongo-class battlecruisers fast battleships?

The label reflected refit differences, translation, and operational need: Japan required capital ships fast enough to work with its unusually quick cruiser line, while the Yamatos were too slow.

The classification is debated partly because Japanese terminology does not translate neatly into English. The four refitted ships also differed in internal subdivision: two are said potentially to meet the fast-battleship description, while two may not. Operationally, the IJN wanted capital ships able to work with its fast cruiser line. Yamato's 27-knot speed is judged insufficient for that requirement. On that fleet-relative definition, a Japanese fast battleship would need at least 29.5 knots.