The economic system of the Roman Empire was slavery. The means of production were people owned as property. Compared to the Feudal system that followed and later the capitalist system, this system is extremely inefficient, riven with contradictory forces and not amenable to innovation.
In all class based systems, including our own, there is a tendency for the ruling class to develop into a parasitic class, living on the wealth of others, but delegating the management of the whole process to to someone else.
Slavery, both modern and ancient is particularly prone to this tendency. Look at the numbers of slave owning families in Britain during the 18th and 19th centuries. They owned slaves and plantations in the Caribbean without the slightest notion of how they were run. They just lived on the wealth. They therefore had not slightest interest in innovation and increasing production and the overseers, bailiffs and managers appointed to run the plantations and manage the slaves had not either.
The slaves in the Roman latifundia who exploited places like Sicily, Egypt and Anatolia to produce the food for most of Italy and Rome and Constantinople used basic methods, with basic traditional tools. Given a certain number of slaves there is a limit on their productivity. The Roman ruling class was particularly parasitic, paying no attention or interest to the managing the system they benefitted from. The Empire had to use vast amounts of its surplus wealth to maintaining a very large standing army, road building for the army and for trade and the wealth of the cities.
The only way to increase production was to expand the empire, more land more slaves. The trouble was more land, needed more armies to protect it.
By 3rd century, the Empire had reached its limit and began to stagnate. Add to that, increasing pressure from migrating peoples of the east and bouts of plague epidemics that reduced the work force. Stagnation set in. Diocletian passed an edict making all trades hereditary, from cobblers to blacksmiths. This was sign of deep stagnation in society. Soldiers became part time, farming small holdings at the same, this of course made them less well trained, less committed and certainly reluctant to travel very far.
The stagnation was particularly pronounced in the west, so that fell in 457. The east of empire was always far more developed, all the major cities and centres of production as well as trade managed not only to carry on but to to thrive until Islamic expansion put it under pressure. The Empire, quite falsely labelled Byzantine (it was the Roman Empire) by western historians in 19th century, survived until 1204, and had a shadowy existence until 1453.
Feudalism being a class system based on landholding was still subject to the tendency of ruling class parasitism, but left slight more room for innovation. The system allowed peasants or serfs to work on their own land (usually communally) 3 days a week and to work for nothing on the lord’s demesne 3 days a week. The peasantry embraced ways of increasing their land’s production. For this reason, the horse harness, a key invention, which enabled a horse to pull a heavy plough instead of the light oxen hauled scratch plough of Roman times. This increased production so much that it is estimated that that the population of Europe doubled between the 11th and 13th centuries. More land came into production as new villages were created. However, increased production meant increased surplus, this meant more trade, more industry and the slow creation of a capitalist class (and a working class), for whom innovation was inherent. This class would eventually overthrow feudalism. The Black Death reduced the labour force so drastically that lords had to start paying labourers wages to work the land, this was the the death warrant for the lord/serf relationship.