What are some difficulties involved in ruling a large empire such as the Persian Empire?

With no mechanical transport or electronic communication, news and responses was very, very slow. So the empire was divided into 20 provinces each with a Persian governor responsible for internal and external security, taxes and improving prosperity, overseen by the king and his council.

To cope with the different languages, cultures and structures, local government remained in place, overseen by the governor.

Successful decentralisation, the opposite of what is happening today with adverse centralised meddling and ill-effects.