I suspect this is well known by all you "old hands" but it has caused a bit of confusion for me until recently so I thought I would share it with newer users of Movie Studio ( I am using Ms V12 but I believe this also applies to previous versions) . There are a few things you can do to ensure a smooth preview in the preview window when you are editing. First and foremost is of course to use a PC with a powerful multi core processor and plenty of RAM but there are other things you can do. You can ensure your PC is set at its most performance oriented setting in Power Options ( Control Panel) and organise your PC so that Movie Studio application is on a different drive to source video for example. You can also transcode your camera output to a more edit friendly avi format prior to import it to MS. Even having done all of that, there are video fx and transitions that can really make the preview window stutter and make it difficult to see what settings you have applied. There is an answer for this problem as well however. If you create a region enclosing the section of video you have applied the transition/FX to, and press Shift B, the section within the region will render to RAM. If you then replay that section it will run smooth in the preview window. The size of region you can render to RAM in this way is determined by the amount of RAM you have allocated for the purpose. This is an area that has confused me for some time! If you go into OPTIONS/PREFERENCES/VIDEO you will see a box where you can set "Dynamic RAM Preview". It is usually set by default at 200MB. I find that a good setting for most editing and rendering purposes but it will only allow a render to RAM for a region of about 5 seconds or so. If you increase it, you can render to RAM, longer selected regions. That appears to be the only reason to change that setting. I have experimented and found that the default setting works on my PC for the majority of editing giving a smooth preview window ( given that I have implemented all the other "performance" steps). If I increase the setting above 500MB it adversely impacts on the smoothness of my preview. So for the initial steps in editing I leave it at the default. When I am then creating a complex title, or transition etc I increase that setting to 2000MB ( I have 16GB RAM) to give me reasonable sized regions I can render to RAM. When I have finished doing those specific elements I return the setting to default..........and certainly have it set at default for rendering ( if you leave it set high you increase rendering time). Hope that helps a few people.