Ultrafast plane-wave ultrasound imaging is a versatile tool which has become increasingly relevant for blood flow imaging using speckle tracking, but suffers from a low signal-to-noise ratio (SNR). Cascaded dual-polarity waves (CDW) imaging can improve the SNR by transmitting pulse trains, which are subsequently decoded to recover the imaging resolution. However, the current decoding method (in the time domain) requires a set of two acquisitions, which introduces motion artifacts that result in incorrect speckle tracking at high flow velocities. Here, we propose a frequency-domain decoding approach that decodes acquisitions independently. Experiments using a disk phantom show that frequency-domain decoding of a four-pulse train achieves an SNR gain of up to 4.2 dB, versus 5.9 dB for conventional decoding. The benefit of frequency-domain decoding for flow quantification is assessed through experiments performed with a rotating disk phantom and a parabolic flow, and through matching linear simulations. Both CDW methods improve the tracking accuracy compared to single plane-wave imaging. Timedomain decoding outperforms frequency-domain decoding in low SNR conditions and low velocities (≤0.25 m/s), as a result of the higher SNR gain. In contrast, frequency-domain decoding outperforms time-domain decoding for high peak velocities in imaging of the rotating disk (1 m/s) and of the parabolic flow (2 m/s), when significant scatterer motion between acquisitions causes imperfect time-domain decoding. Its ability to decode individual acquisitions makes the proposed frequency-domain decoding of CDW a promising approach to improve the SNR and thereby the accuracy of flow quantification at high velocities.