Not 100 percent this is legit.
Aight. So, the feat. An enraged Luxray fires a bolt of lightning at the protagonists, and Dusknoir runs up and eats it before it hits them.
https://youtu.be/eqsi7n0VDF0?t=6m6s
I'm assuming it's true lightning. Even the weakest electric move, Thundershock, is seen to come from above straight down as lightning in the MD series. And Luxray was enraged, charging up its strongest attack, and creating an electric field while doing it.
I'm feeling lazy, so given perspective, the distance between the beam and the protags is approximately Chimchar's height. Actually...nvr mind. It's simple
Chimchar is .508 meters tall, and 57 px. The distance is 67 px.
x/.508=67/57, and solving for x gets me .597 meters
Dusknoir travelled 313 px to Chimchar's 57 px in height.
x/.508=313/57, and solving for x again gets me 2.7 meters.
.597x=2.7. Solve for x a third time gets me 4.522613 times faster than the bolt.
Which puts Dusknoir at a solid 1989949.72 m/s.
Mach 5801.6
Probs an outlier, but I wanna discuss it anyway.