Prime v secondary, to me, is a combination of location, property grade and tenant covenant strength.
Argument is that the sheer weight of money sloshing around in the markets during boom bid up values of secondary properties and narrowed the yield gap with prime = yield compression.
He put up graphs that showed the last couple of boom / bust scenarios - yields in secondary continued to lengthen after prime yields had stabilised until the yield gap reflected an appropriate risk premium. The lag was consistently about 1 year.
This was in commercial and retail sectors - same probably true for industrial.