Metrics of system performance are used to assess the abilities and safety of x-ray imaging systems. The detective quantum efficiency (DQE) is used as a measure of "dose efficiency" but, when applied to fluoroscopic systems, requires a measurement of the temporal modulation transfer function (MTF) to account for the effects of system lag. It is shown that the temporal MTF is exposure-rate dependent, and hence must be measured under the specific exposure conditions of interest. We develop a small-signal approach to temporal MTF measurements using a semi-transparent moving slanted edge. Using an x-ray image intensifier-based bench-top system, we show that there is a 50% overstatement of the DQE when not properly accounting for lag. The small-signal approach is used to calculate a lag-free fluoroscopic DQE that agrees with a radiographic DQE measurement under the same exposure-rate conditions. It was found that the temporal MTF did not change within measured precision over normal fluoroscopic conditions, and the radiopaque falling-edge results were consistent with the small-signal temporal MTF. This approach could be implemented in a clinical setting with access to raw (linear or linearized) fluoroscopic image data and could be generalized for use on pulsed-exposure systems.