When starting a new project, click OPTIONS | DISK tab and Clear Unused Cache Files. You can use this at any time during editing.
If returning to the most recent project do not clear the cache.
Be sure that the caching disk (usually C:\) has ample free space, or direct the cache folder to a drive that does, at the bottom of the same screen.
The cache contains small files that speed up editing. When loading content, allow the green cache line to complete.
Save Project File AS routinely, giving it a unique # or name, so as not to overwrite prior saves. Using Save Project Files (no AS) will overwrite.
That AS choice is available via the MENU | FILE drop-down 
or, when the HOME tab is active, by clicking the chevron to the right of the Save Project button. 
If Videopad becomes sluggish, try the 64-bit version, which is capable of handling more complex and lengthy projects.
