The question of how much mankind advances during which era depends on whether or not you accept the new novels' retconned history of the Butlerian Jihad. Frank Herbert had the major tech/cultural advances happen over a logical time frame of several millennia -- and then it was thousands of years again until Paul Atreides came along. The KJA/BH novels have Norma Cenva invent everything significant within a single century, which blatantly contradicts Frank Herbert's established canon.
So in the real Dune history, Earth was not devastated by nuclear bombs (at least not when KJA/BH said it was, else the people who hammered out the details of the Orange Catholic Bible would not have had anyplace to do their work). Technology was invented by more people than just Norma Cenva, and over a greater, more realistic time frame. The Bene Gesserit existed long before the Jihad.
Actually, it makes sense that technology/major Imperium-wide cultural advances would have plateaued during the Imperium. That's what tends to happen with every Empire, from Earth's Bronze Age to the far future. People have what they need, their technology works well and serves its purpose, and there is no real necessity to push forward. It usually takes a major event or breakthrough to sufficiently shake up society enough to cause really noticeable change. In our own history, that happened with inventions such as the printing press. In Dune history, it happened with the discovery of foldspace technology and understanding of the myriad benefits of consuming the spice.