Who is slower than Couts?
Among forwards, JVR.
That's probably it.
Laughton, Pitlick, Voracek, Farabee and of course TK and Frost are definitely faster.
Giroux, Raffl and Hayes are probably faster, though not by a huge margin.
Among defensemen, Braun and maybe Hagg (though he's not a bad skater, his issues are more mental).
Note that the Flyers aren't putting many slugs on the ice anymore, so being one of the slowest players means you're more around "average" than past years.
What this team lacks is high end speed, only TK and Frost have the jets to freeze defenders.
Couts has great body control and hands, he can skate through crowds and maintain possession, and just as important, control of the puck (Voracek and Hayes are good at maintaining possession with their bodies but don't have as good of hands in a crowd).
And he takes great angles, which reduces how far you have to skate.