Found this video and thought it was interesting. Shows that Dubois is deceptively fast, especially for his size.
For straight line speed without the puck he skated a 4.061 (second half of the video around 13 minutes, when team Orr takes the ice). Compare that to Tkachuk who had 4.584, Pascal Laberge who had 4.167, or Jake Bean who had 4.127 and I think any skating concerns can easily be put to rest. After the top three, he is easily the most well rounded player in this draft (Tkachuk may have better hands and be smarter in front of the net but his skating holds him back in my opinion).
Kyrou had a 3.926 and Chychrun had a 3.977 so some of the people beating Dubois aren't too slow either (He finished 6th or 7th overall for speed between the two top prospect teams).