I think you may be confusing two different things here.
In Cp5, preloading 100% of the file does not necessarily mean you will get instant response when you jump from an early slide to one a lot later in the project. This is because even when the project is fully loaded up, Cp5 needs to 'render' all of the slides in between the jump slide and target slide BEFORE it will load the target slide. So you can get a delay ranging anywhere from a split second up, depending on the size of the project. If you want to test this, try timing the difference in delay when jumping to a slide 10 slides down the timeline then 50, then 100, then 200. The delay will be progressively greater.
This delay issue has been mitigated to some extent in Cp6 and now Cp7 because they take a different approach to how objects are added and removed from the timeline.