I've never seen a low-speed digital circuit! The logic gates switch just as fast whatever rate they are switched.
Its important to have good decoupling, and that is more important that a continuous ground usually. If you are routing high current fast-switching signals then you must be more careful or the return currents will prefer to flow in nearby signal wires.
One approach is to explicitly route your ground network, and then pour ground - that way you know all grounds in the circuit are directly connected.