I think this is where it gets hard, because the reality is that the collision would not be perfectly elastic and some energy would be lost as heat within the balls. But onlooking this up I realise I have made a mistake. A shock wave can travel faster than sound in a medium. https://www.britannica.com/science/shock-wave So that I think will be the explanation of what happens. Instead of a sound wave within the balls, you will get a shock wave, which travels faster and can transmit the necessary kinetic energy to allow the previously stationary ball to acquire the same velocity as the incoming one, even though it is faster than the speed of sound in the material of the ball. But not much of this will apply in the case of QM entities. Due to the uncertainty principle (or, to put it another way, the wavelike nature of these entities), we won't be able to define the details of the energy transfer process, so I don't think we can talk of a speed of energy transfer in such a case. The boundaries of the objects are fuzzy (we have to deal with the "cross section" as a measure of probability of a collision taking place), the repulsion between them will be due to things like electric charge or the Pauli Exclusion Principle and will build up in a certain way as they approach......it all feels very hairy........but no doubt a particle physicist would be able to comment a lot better than I can.