Frequency Modulated Continuous Wave (FMCW) automotive radar using stretch-processing usually relies on a fast-chirp signal model which assumes range and Doppler decoupling between fast and slow-time dimensions. To achieve high range and speed resolutions, a large bandwidth and a long coherent processing interval are used. However, if multiple range resolution cells are crossed during the integration duration, fast-chirp signal model assumption becomes inaccurate, resulting in misfocusing in both fast and slow-time dimensions. This misfocusing leads to the misestimation of target parameters and a loss of coherent integration. In this paper, Range Cell Migration (RCM) processing loss is studied and illustrated using a FMCW signal model. Furthermore, a windowing in both dimensions is proposed to mitigate this loss.