tVALID is the slave time response to output data after detecting a QSCK edge.
For a QSPI slave device with tVALID (or tV) = 12 ns, fQSCKmax = 66 MHz at VDDIO = 3.3V.
For a QSPI Flash memory device with tVALID (or tV) = 6 ns, the formula returns a value of 112 MHz. In worst case conditions, this exceeds 66 MHz, which is the maximum allowed frequency of the QSPI master. In this case, the limitation is due to the controller and not the slave.