Regarding the speed issue, we have to prepare ourselves for the notion that some of this is by design. Offensively we don't have an Andre Johnson and we lack some depth but Duke, Dorsett, Hurns, Waters, and Coley is a foursome that we can take as upper echelon IMO. We're missing a dominant force and depth but that's ok given where we were.
The D is a different story. We definitely need more explosive players at safety no question. But up front we're a different team philosophically. We've always been built on undersized quick twitch players that penetrate and disrupt. Our last defensive system (and many prior) was all about getting upfield, disrupting the backfield, and pressuring the passer. We now have a philosophy built upon size, controlling the line of scrimmage, and keeping things in front of us. We substitute in some explosion but our base defense emphasizes size and physicality--275 lb ends, an interior plugger, and sizable backers. Perryman is a 6' linebacker that they ASKED to get to 240 lbs. When you think of our more explosive players, where will they play if they play consistently? McCord will probably be asked to get to 260 lbs and replace Shayon Green I think--the tradeoff will be some of his speed likely. Muhammad has no position next year he is unlikely to play except in pass rushing situations. Jermaine Grace can play nickel but I don't see him playing in our base defense until he's 230+ lbs. None of this is to say that we can't get more explosive defensively (especially in the secondary), but we don't look like we're interested in being the type of defense we've been historically. That's for better or worse it's all about the results.