Playing all the games doesn't mean you don't get small injuries and just play through them. Just watch Alzner's play from 3 seasons ago and watch him now and you'll see the toll blocking shots and playing physical can have on a defenseman's physical atributes and skating speed (there are probably several reports about him not even being able to open pickle jars at home because of damage to his hands), despite not missing a ton of games. No concrete source, but watch Muzzin's stride (and position while skating) now compared to what it was in something like 2015 and you'll notice that they're a bit different from each other. That, to me, screams lower-body injury (or musculo-skeletal/spinal problems, who knows).
As for Muzzin not being slower, I completely disagree. Although he hasn't been all that fast from the start and relied mostly on his awareness and positioning (as well as Doughty bailing him out when all else failed) to make plays, he has slightly more trouble now reaching the puck carrier and pinning him alongside the boards to neutralize plays than he did before. In a straight line, skating from one side of the rink to another, yeah, there isn't THAT much difference, or you could say it's subtle, but where Muzzin's loss of speed is most noticeable to me is in his first couple steps, his acceleration.
Anyways, despite that Muzzin is still a premium asset for the LA Kings and a really, really good defender, one that I'd be thrilled to have on the Canadiens.