What version of Videopad is being used? What are the sizes and format of source files - HD, 2K, 4K?
Try toggling Hardware Acceleration, under OPTIONS | EDITING tab to see if that helps.
Under the OPTIONS | DISK tab, Clear Unused Cache Files. Also shut down other programs that may be running the in the background.
If performance is slow after files are cached (completed green lines above thumbnails), take a look at PROXY editing. With very large files it can increase efficiency considerably.