guideline

Use column-first processing of two-dimensional arrays when measured performance indicates.

    type Table_Type is array (Row_Min .. Row_Max, Col_Min .. Col_Max) of ...
    Table : Table_Type;
    ...
    -- Row-order processing
    for Row in Row_Min .. Row_Max loop
       for Col in Col_Min .. Col_Max loop
          -- Process Table (Row, Col)
       end loop;
    end loop;
    -- Column-order processing
    for Col in Col_Min .. Col_Max loop
       for Row in Row_Min .. Row_Max loop
          -- Process Table (Row, Col)
       end loop;
    end loop;

rationale