They were much less behind than it is now (1-2-5 years max). The problem is that leadership of USSR wanted to take advantage of using existing software - hence

They were much less behind than it is now (1-2-5 years max). The problem is that leadership of USSR wanted to take advantage of using existing software - hence demanded 100% exact copy of hardware (starting from IBM-360). By many these actions were considered a treason, especially in retrospective. There were independent developments, they worked but never went widespread (because they needed an "order" from the state).