  Three key metrics for readout systems in quantum processors are measurement
speed, fidelity and footprint. Fast high-fidelity readout enables mid-circuit
measurements, a necessary feature for many dynamic algorithms and quantum error
correction, while a small footprint facilitates the design of scalable,
highly-connected architectures with the associated increase in computing
performance. Here, we present two complementary demonstrations of fast
high-fidelity single-shot readout of spins in silicon quantum dots using a
compact, dispersive charge sensor: a radio-frequency single-electron box. The
sensor, despite requiring fewer electrodes than conventional detectors,
performs at the state-of-the-art achieving spin read-out fidelity of 99.2% in
less than 6 $\mu$s. We demonstrate that low-loss high-impedance resonators,
highly coupled to the sensing dot, in conjunction with Josephson parametric
amplification are instrumental in achieving optimal performance. We quantify
the benefit of Pauli spin blockade over spin-dependent tunneling to a
reservoir, as the spin-to-charge conversion mechanism in these readout schemes.
Our results place dispersive charge sensing at the forefront of readout
methodologies for scalable semiconductor spin-based quantum processors.
