The ZBTT has nothing to do with zero "slack", if introduced ZBTT is based on Zero Sum Game theory, which means at a destination if 100 trains were sped up, there will be other 100 trains which need to be slowed down to help those 100 trains so net is 100-100=0. Now this is not for NDLS alone, its per destination so we will see trains slowed down in intermediate sections as well, while doing so some trains will get insane time slots affecting passengers while others will retain their VVIP status.