[nsilvestri]

You can derive it yourself. Dimorphos's orbital eccentricity is pretty low, meaning you can use:

mean_orbital_velocity = 2 * pi * semimajor_axis / period

As for the orbit around the sun, the parent body Didymos is about 100x more massive, so any change in the center of mass from this impact will have, in the short term, negligible effects on their orbit around the sun.

[sega_sai]

No, that is not correct thing to use, as the semimajor axis will change as well. What is needed is the third Kepler law. The semimajor axis will be period^(2/3). And then speed will be period^(-1/3). Thus if period was changed here by roughly 5%, the speed changed by 1.6%