Weapons in Splatoon 3 categorized as rollers, sloshers, brushes, or splatanas have a stat labeled handling. What does this handling stat do?
Looking at Main weapon data for Splatoon 2, it should be related to Mobility.
Mobility: Movement speed while attacking or rolling or holding a charge. Measured in DU per frame. DU refers to the game's internal distance unit. There are 60 frames every second in Splatoon.
As you can imagine heavier weapons have a lower speed.
So Handling in Rollers, Inkbrushes, Sloshers (and now Splatanas) is the mobility while in use. Specifically, it's the movement speed, turn speed, and turn radius if one is moving and turning simultaneously.