I don't think slowing them down is the answer.
For building muscle, I think you need to get tricky. Bryce Lane's "B-Circuit" concept might be useful.
For those who don't know, a B-circuit consists of:
1. Explosive, short range movement (jumps for height)
2. Full range explosive movemetns (squat, then jump for height)
3. Deliberate, "tensed" slow movement (air squat with you actively engaging the muscles to pull yourself down and to resist going back up)
Even those bodyweight masters, gymnasts, don't get all that big or heavy, and they do the big dog of bodyweight exercises. I think you hit a point, sometimes very quickly, where you need to add some kind of external resistance.
It's kind of a "right tool for the job" thing. Bodyweight exercises will work for a while, especially for those not used to doing them