Topcliffe Kid, because there is no need with ADS-B. Its typically transmitted as part of 1090Mhz Extended Squitter. The box is interrogated, then sent back straight away. There is no latency. Same as ACAS (TCAS II) signals, same as Mode S.
The difference with ACARS is that there are significant latencies from how the signal is transmitted over VHF (POA, VDL Mode 2 or 4) or SATCOM, handled by a communications service provider, then re-transmitted to either an ops centre or ATC centre. There is not direct connection, so you need a timestamp in order to determine when exactly the message was sent.
This
doc does a good job of explaining how busy the 1030Mhz and 1090 Mhz frequencies are and what is involved with Secondary surveillance and the like