No mention of burnout? If a developer becomes suddenly unproductive burnout is often the culprit. Also, in my experience another culprit is: your developers a

No mention of burnout? If a developer becomes suddenly unproductive burnout is often the culprit. Also, in my experience another culprit is: your developers are slow because you aren't willing to pay them market rates or show them respect so even though their conscious mind wants to work their butt off for you, they just can't do it because moral is so low.