@matonanjin an important part of using proxy files created by Movie Studio is the Preview Quality. In the screenshot @j-v provides (index help) it is cut off. If you preview in 'Draft' or 'Preview' quality the proxy file will be used for previewing, however if the preview quality has been set to 'Good' or 'Best', your original source media will be used. Switch your preview quality to 'Good' or 'Best' to view at better quality, but switch it to 'Draft' or 'Preview' quality for smoother playback.