The Time feature considers the entire clip on the Timeline, not just the portion you select, resulting in frames from parts of the clip that you did not choose appearing in the speed-adjusted section.
To work around this issue, you can export the project section to adjust the playback speed to a separate video file. After you change the playback speed of the individual video, import it into your original project as a new clip.
1. Export the section of your project that you want to adjust the playback speed.
1. Right-click the clip to export, then select the Properties option.
2. In the Properties window, scroll down the table to the Frame Number property.
3. Right-click the value next to Frame Number (default value is "None") and select "Timeline."
4. Move the play-head (the red vertical line on the Timeline) to the first frame of your selected clip.
5. Take note of the number in the upper left-hand corner of the preview screen, the first frame.
6. Move the play-head to the last frame of your selection.
7. Take note of the number in the upper left-hand corner of the preview screen, the last frame.
8. Move the play-head to the first frame of your selected clip.
9. Right-click the value next to Frame Number and select "None" to turn off the frame numbers displaying.
10. Click the "Export Video" button (the small red circle icon) in the toolbar at the top of the Main window to open the Export Video window.
11. Enter a new name for the exported section into the File Name: field.
12. Enter a destination folder for the exported section into the Folder Path: field.
13. On the Simple tab:
1. Review the Target, Video Profile, and Quality settings.
Ensure the Target, Video Profile, and Quality Settings match the source clip.
14. On the Advanced tab:
1. Change the Start Frame to the clip's first frame.
2. Change the End Frame to the clip's last frame.
15. Click the Export Video button to export only the selection of your project.
2. Create a new project and import only the clip to adjust the playback speed.
3. Change the speed of that clip, then export it as another video file.
4. Import the recently modified clip to your original project and add it to the Timeline.