We use fast gate-voltage pulses and multiplexed RF-reflectometry charge sensing to coherently manipulate and read out multiple singlet-triplet spin qubits, within a 2D array of 12 gate-defined quantum dots in GaAs. By measuring the qubits' responses to precise timing changes of control pulses generated at room temperature, we characterize in situ the synchronization of control pulses with sub-ns resolution. These techniques are useful for calibrating the effective delay in cryogenic transmission lines for silicon quantum processors.