The familiar number comes from mid-twentieth-century tests in which a hooked fish stripped line from a reel and the rate was converted into a speed. That method measures a panicking animal against drag from a line, over a short burst, and it was never repeated under controlled conditions.
High-speed video of sailfish attacking sardine baitballs tells a different story. Published analyses of striking sailfish recorded bill-tip and body speeds far below the legendary figure, in the region of a few metres per second for the strike, and biomechanical arguments about muscle power and cavitation set a ceiling well under 110 kilometres per hour for sustained swimming.
What sailfish demonstrably do is more interesting than the number. They hunt cooperatively around baitballs, taking turns to approach, and they tap and slash with the bill to injure individual sardines. They also flush with bright colour changes during these attacks, which may signal to other sailfish and reduce collisions.