Motion's caching will utilize your computer's RAM, taking as much memory as defined in the Project Cache preference.

To view or change this setting in Motion, do the following:

Choose Motion > Preferences.
Click Cache in the toolbar to reveal the Cache pane.
The Project Cache setting represents the percentage of your computer's total memory that you wish to use. The larger the number, the more memory will be available to Motion to cache frames, resulting in a higher likelihood of consistently smooth playback.

The Project Cache setting is retained as global to Motion; when you close a project, the memory is still reserved while Motion is running. This memory reservation may affect the performance of other applications that are running at the same time as Motion.

Considerations

Depending on your work style, there are a number of approaches to balancing the memory requirements of Motion and other applications that you run.

If you run many applications simultaneously, try a lower Project Cache setting, so that memory will be available for other applications as well as Motion.
If you prefer a high Project Cache setting for increased performance while using Motion, you can quit Motion when you are finished, so that the reserved memory will become available to other applications.
Adding more physical RAM to your computer is another way to allow Motion to have a relatively large memory reservation, with a lower Project Cache percentage setting. This allows a proportionately larger amount of memory to be used by other applications.