The dynamics of polarization evolution and rotation in 0.68PbMg1/3Nb2/3O3-0.32PbTiO3 relaxor ferroelectric thin films are studied via in operando synchrotron-based X-ray diffraction with AC electric fields. A frequency-limited suppression of polarization rotation was observed above ultrasonic frequencies (≳ 20 kHz). The nature of this suppression is informed by scanning transmission electron microscopy in the zero-field state, where a high density of nanoscale, low-angle domain walls was observed. In combination with switching dynamics studies, the results suggest that the suppression of polarization rotation at ultrasonic frequencies is due to the large activation field needed to move the domain walls when the polarization rotates between different monoclinic phases. These results are critical in understanding piezoelectric relaxation phenomena in relaxor ferroelectrics.