Actually there are certain things (e.g. sounds, random creature spawns and scripted events) that we don't preload because they're hard to predict, so using a slow drive can lead to momentary framedrops as things are loaded in. That doesn't happen very often, so it'll be more of a 'microstutter' than a noticably reduced framerate. Using an SSD will alleviate these framedrops (aside from reducing the chance of something not being preloaded on time, which can also lead to framedrops and/or waiting time). If you don't have an SSD, you can put files on a ramdisk (assuming they fit in RAM) to make things faster.Any stuttering you experience is not related to having either your game files or OpenMW on a SSD or not. A SSD simply leads to a reduced loading times, not an actual performance boost.
There could be a small benefit in loading time to putting OpenMW itself on an SSD as well, but that will only be relevant during game startup and never in-game. In any case, the game files will make a much larger difference.
