King_strut, did you ever find the problem?
I have the same issue with a 2012 Tahoe 5.3. First start of the day is usually normal, almost every start for the rest of the day has a long start, no tach, and loss of power. The only exception is if it sits for 4 or 5 hours, then maybe it will start normal. I have been trouble shooting for a couple of months and find nothing.