We show that the combination of a free-running 24 kHz supercontinuum fiber laser with a kHz femtosecond laser renders a sensitive and easy to implement transient spectrometer. It can directly extend the observation range of a standard broadband femtosecond spectrometer from the usual limit of a few nanoseconds to 1 millisecond and with the additional use of a chopper to seconds. The transmission signal from all supercontinuum pulses is recorded and assigned to the proper delay time by data post-processing. To demonstrate the performance of the device, the relaxation dynamics of Nd:YAG is measured.