We demonstrate an efficient phase-retrieval method, allowing the recovery of features of dynamic objects that are sparsely varying - i.e. when the difference between two consecutive frames is small. The method uses redundancy in information between similar consecutive frames to recover the features more robustly than current phase-retrieval methods, and necessitates a considerably smaller number of measurements. Both of these features directly lead to shorter acquisition time, paving the way to coherent diffractive imaging of fast dynamic processes. Numerical simulations show a possible 100-fold improvement in temporal resolution over existing methods.