Boxes are the simplest only being beat by spheres, but if you add a lot then you might as well just use fewer convex.
Boxes are ok if your object fits the shape, but if you have to start box modelling with many of them that sort of defeats the point.
Box collision will check intersections based on a math formula of the origin and width, height and depth which would on its own be faster than a convex check.
But if you have many to do it adds up.
Convex on the other hand has to check if another primitive is within it’s volume of vertices. More expensive to do but if the collision doesn’t have to many verts and follows the shape well the it beats out many boxes.