The measurement speed is an important parameter of terahertz time-domain spectroscopy (THz-TDS) instruments. To improve the measurement speed of the spectrometer we need to increase the scanning speed of the delay line. In this paper, we study the influence of the scanning speed, the time constant of lock-in amplifier and the sampling rate on the signal quality. The results show that, when the time constant equal to 10 ms, increasing the scanning speed does not cause significant changes in the amplitude of the signal. But when the time constant equal to 30, 100 and 300 ms, with the increasing of the scanning speed the signal amplitude decreases rapidly. Therefore, the time constant should be set as small as possible to avoid deterioration of signal quality. On the other hand, higher the scanning speed is, fewer data points are collected with a same time-domain length. Therefore, when the scanning speed increase, not only the time constant should be reduced, but also the sampling rate should be increased to avoid the distortion of signal waveform caused by the number reduction of data points. The conclusions can provide a reference for improving the measurement speed of THz-TDS instrument.