Quantum measurements of large qubit ensembles are often performed indirectly by coupling the ensemble to a detector and subsequently measuring the detector. Despite the importance of rapid collective readout, it remains unclear what determines how the readout time scales with the number of qubits N. Here, we first establish a benchmark N^(-1/2) for a broad class of detectors without ultraviolet high-frequency modes. We then show that the detector with unbounded high-frequency modes can surpass this benchmark and yield a characteristic time scaling N^(-1/(2-ν)) for 0 < ν < 2, where ν characterizes the spectral structure of the detector. Our results establish high-frequency detector modes as a resource for achieving a scaling advantage in collective quantum readout, with the detector spectrum directly controlling the scaling exponent of the readout time.