I'm sure Miller knows his own body better then anybody and if he feels he's already fast and flexible enough, and that adding some bulk and power will help his game, who are we do challenge that.
I remember when Kobe Bryant starting hitting the weights pretty hard and you thought he might lose some of this quickness to get past defenders and all that stuff. Didn't seem to hurt his game. If you really think about it, adding muscle really doesn't make you slow. Just go look at some of those all world track stars, the sprinters. Some of those guys are really built. They got pecs, biceps all that stuff. I think Von Miller understand that he's not really going to be dropping back in coverage all that much. He's a LOS guy and a pass rusher. Adding some chest and arm mass with power isn't going to affect his quick twitch muscles. If anything, I think another reason he is doing this is not just for performance, but for durability too. He could simply feel like as the season wears on, he gets too small and feels worn down a little. This could help.