If I was to guess, I would say that the resources for the system or the grpahics card are in high demand, so to process the necessary information there is a delay...
Each computer will have different responce times, depending on the hardware of the system, so there is no way to specifically state that is or that will help. If you have Movie Edit Pro MX, then I would recommend using the Pre-Render for the areas that have things going on that might slow down the process.
Hi, RAM, CPU speed, OS, available disc space, number of open applications, etc will have a huge impact on how long it takes to load, pre-load, render, or burn a video. You have not posted any of your system specs, but I can tell you, if your are running 2 GB of RAM which is pretty normal, and you have other applications running, you will see very slow loading, pre-loading, rendering, and burning. If you are running windows XP or Vista, neither OS is very video friendly, XP especially. 2 GB of RAM is probably the minimum that you need to have for MEP to run, but that 2 GB needs to be available to the application if you want decent results. You would be much better off with 4 or 6 GB or even more RAM if you want really fast response. I run Windows 7 64-bit with 6 GB of RAM on a Quad Core processor and I often feel that I am not getting the speed that I want. Food for thought.
Just to add to what John and George have said - your antivirus prgram can also have a significant effect on performance - particularly the two popular heavyweights which seem to come pre-installed on most new computers.
From personal experience I will not use the two popular heavyweights because they are resource hogs and can significantly slow a machine down.