"There is a fifth force which shaped religious belief, but its effect is so universal and profound that it deserves to stand alone...it deserves to be written thus: SPACE TRAVEL!"unknown[src]

Space travel played a major role in the evolution and expansion of humanity throughout the known universe. Two forms of space travel existed: faster than light space travel, and conventional space travel.

Supposed early draft for Jodorowskys Dune

For several thousand years, faster than light travel (FTL travel or space-folding) was conducted exclusively by the Spacing Guild, using Spacefolder vessels piloted by Guild navigators that folded space-time and moved almost immeasurable distances in the blink of the eye.

This form of travel, while extremely expensive, was also not safe as one in ten ships that used space folding engine disappeared, at least during the early years of the technology's use before the advent of Navigators. It was utilized for both commercial and military purposes. Space-folding made use of two key factors:

Eventually, at some point between the fall of the Atreides Empire and the discovery of the Dar-es-Balat hoard, Ixian navigation machines broke the guild monopoly on foldspace by providing a means of safely navigating foldspace without a navigator.[1][2]

The old FTL conventional space travel was used mainly for travel within the confines of a star system (not for interstellar travel). However, before the discovery of the new faster-than-light travel method, it was also used for long-distance space travel. The old method was described as "outracing photons". Even after space-folding became the primary means of interstellar travel, many Imperial warships still kept their old FTL drives as an alternative to the much faster but less reliable Holtzmann engines.

A calculation for velocities obtainable with old FTL conventional space travel can be made from the book "The Butlerian Jihad" by Brian Herbert and Kevin J. Anderson. Before the Battle of Earth, the unified Armada is stated to have gathered at Salusa Secundus. This planet is stated in this wikipedia to be located in theGamma Waiping system which is about 130.8 light-years form Earth, again according to this wikipedia. In the book"The Butlerian Jihad" Xavier Harkonnen states that the Armada takes over a month to reach Earth while traveling at its maximum sustainable speed. Using terrestrial time periods (days, weeks, months) for simplicity's sake, we get1,591.4c for a month,1,136.72c for six weeks, and795.7c for two months (c being equal to the speed of light).

The connection between faster than light travel and the Holtzman Effect is not explicitly mentioned by Frank Herbert. It is a connection made in the prequel novels by Brian Herbert and Kevin J. Anderson.

In the 'Legends of Dune' trilogy, the pair describe the time shortly before and during the discovery of space-folding. In these works the discovery of space-folding is attributed to Norma Cenva, who goes on to become the first prescient folded space navigator. Prior to this, although described in 'The Machine Crusade' as "outracing the old faster than light method", vessels still took weeks or months to cross between even the closest stars.

Excerpt from:

Space travel | Dune Wiki | Fandom