Speed’ is acceleration. And even if you cant legit speed combo you should still realize that the centrifugal force during a brain twister also reflects the yoyos accelerative capability. In this case light yoyos with less rotational moment of inertia is faster since to motion of the yoyo is largely circular and the force on your hand is continuous. Such yoyos require less centripetal force from your finger to be swung at constant high speed/ pulled to reverse its direction. A yoyo with large rotational moment is difficult to control (=accelerate/decelerate/keep in uniform circular motion). Shinya Kido pulls the yoyo in the radial direction to accelerate it instead of bouncing it on a string. This motivated the shift from the rim weighted sleipnir to the center weighted krown.
Acceleration by string hits is a different matter. The forces on your fingers change so fast that you could easily overreact with too great a force and kick the yoyo off the string framework. In this case a yoyo with medium high total weight and rotational inertia is ideal for fast play (!=fastest), where a slightly higher centripetal force lets you keep track of the yoyo and a solid weight gives you reliable feedback on your hand movements at subconscious speed. Mickey’s yoyos fall under this category as his tricks often employ string hits to change the yoyos direction.