  We study the speed/fidelity trade-off for a two-qubit phase gate implemented
in $^{43}$Ca$^+$ hyperfine trapped-ion qubits. We characterize various error
sources contributing to the measured fidelity, allowing us to account for
errors due to single-qubit state preparation, rotation and measurement (each at
the $\sim0.1\%$ level), and to identify the leading sources of error in the
two-qubit entangling operation. We achieve gate fidelities ranging between
$97.1(2)\%$ (for a gate time $t_g=3.8\mu$s) and $99.9(1)\%$ (for
$t_g=100\mu$s), representing respectively the fastest and lowest-error
two-qubit gates reported between trapped-ion qubits by nearly an order of
magnitude in each case.
