While slow motion has become a standard feature in mainstream cell phones, a fast approach without relying on specific training datasets to assess slow motion video quality is not available. This manuscript proposes a fast and generalized no-reference objective metric based on temporal loss in modulation transfer function (MTF) to evaluate interpolated slow motion mode in mobile phones. First, a standard chart embodying slanted edges is used to capture a slow-motion video. Second, the edge spread function is extracted from a region of interest in individual slow motion frames. Third, the line spread functions and MTFs are calculated. Finally, reference and interpolated frames are identified. Sharpness loss in slow motion mode is quantified by the MTF area difference between minimum interpolated frame and reference frame MTF scores. The proposed approach is evaluated in simulated and experimentally captured slow motion videos. In experiments, slow-motion videos are captured by moving mobile phones mounted on a motorized linear stage apart from the test chart at a constant speed while keeping the test chart still. The proposed MTF scores of several mainstream cell phones are analyzed and compared.