If you're working on a project which uses a lot of your system's resources, your processor may have trouble keeping up with the preview frame rate. To compensate for this, VEGAS automatically reduces the frame rate to ensure the real-time preview feature keeps working.

If a high preview frame rate is more important to you than high preview quality (e.g. during dynamic RAM previews of key scenes), you can reduce the quality (i.e. resolution) of the preview. Doing this eases the demand placed on the processor so that you can preview your video at a higher frame rate.

The options for setting the preview quality are located above the preview window. 

1. Draw a loop region over the section of the timeline you want to play. To do this, click the slider above the timeline and drag it to the right.
2. Press Shift+B to render the loop region in your computer's RAM.
3. Play your project.

If you want to preview a larger section of your video, go into the options menu in VEGAS, select "Preferences" > "Video" and increase the Dynamic RAM Preview setting.