timelets

Given the huge size of the empire, factors of distance and time determined how closely central government could control the diplomatic activities of its governors on the frontiers.
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Yet this was not the norm.
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Sea travel was largely seasonal and often dangerous. Probably more important, it was highly unpredictable.42 A death sentence from Caligula in Rome for the governor of Syria was three months en route, arriving twenty-seven days after news of the emperor’s death.
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The relative reliability of land communication was the preferred option. Augustus is said to have introduced a system of runners (Suet. Aug. 49), but if it was ever implemented it was soon abandoned. The Principate relied on the imperial post (cursus publicus), a system where those with official authorization (diplomata) could requisition horses and vehicles from either private sources or official posting stations (mansiones).43 It has been estimated that the average speed of this system was about 50 miles a day, although for urgent messages it could have managed up to 160 miles a day.
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The sometimes leisurely nature of diplomacy can be accounted for by the nature of ancient warfare. It was both seasonal, rarely being conducted in the winter, and slow-moving, ancient armies usually only moving at a speed of about 15 miles a day.46 There was often no need for diplomacy to hurry

--- Sabin, et al. The Cambridge History of Greek and Roman Warfare, V2.

The ratio of communication speed vs the underlying events is what matters the most.