This is top speed.
I don't think they track average speed (at least publicly) but I don't think it really makes sense to do so. Different players play different style games.
If a D has a high average speed I can tell you that they're probably carrying the puck up ice a bunch. There aren't many of those guys in this league (at least guys who are good at it.) One of the reasons why I think Schaefer is a clear cut #1 in the upcoming draft, dude is an exit/entry machine and he's got the hands to match.
Exhibit A:
It's a great skill to have in your tool kit (and frankly, one I think we could desperately use) but hardly a necessity. Makar is bar none the best puck rushing D in the league but he's got his warts too - Thats a player who has generally been terrible at defense the last few years and has been average or worse every year of his career except the one year he won the Norris.
Got off topic a bit but back to Fox: His bursts have always tracked below average. He's not rushing the puck up ice and we don't want him to, it wouldn't go anywhere. He passing pushes transition and thats fine, he's very good at it.