  We explore the physical limits of pulsed dynamical decoupling methods for
decoherence control as determined by finite timing resources. By focusing on a
decohering qubit controlled by arbitrary sequences of $\pi$-pulses, we
establish a non-perturbative quantitative upper bound to the achievable
coherence for specified maximum pulsing rate and noise spectral bandwidth. We
introduce numerically optimized control "bandwidth-adapted" sequences that
saturate the performance bound, and show how they outperform existing sequences
in a realistic excitonic-qubit system where timing constraints are significant.
As a byproduct, our analysis reinforces the impossibility of fault-tolerance
accuracy thresholds for generic open quantum systems under purely reversible
error control.
