ADS-B from the SSR Transponder is not time stamped by the aircraft. The receiving station adds the time stamp. There is a mechanism for allowing the receiving station to compensate for various latencies in the aircraft system (known as the T-bit). However this is seldom implemented in current installations. Consequently velocity calculations from the ADS-B position reports alone should not be considered reliable.
HOWEVER ADS-B does transmit the aircraft's own calculated velocity (usually ground speed) at the same rate that it transmits position (twice per second). This velocity should be considered perfectly reliable but I am not sure if Flight Radar carries the velocity component.