You should get better results by first converting video files to low resolution, then using those proxy files - saved to a different folder than the originals - for your edit.
When the project is complete, save the final. Then substitute the higher-res files for the export. To convert, use a tool like NCH Prism.
Two replacement methods...
Right-click the file/s in the bin, individually, and CHANGE FILE. Find the original file and click on it.
Or, if the proxies have been given exactly the same names as the originals, use the PC file manager to replace them in the folder that VP uses, while retaining the proxies in their own folder.
If, during the edit, there is a global effect such as Black and White, it might be better to export, as above, then add that effect to the file that results. That file will be a single clip, of course, and cannot be deconstructed.
Under OPTIONS|EDITING tab, toggle the Hardware choice, to see if that improves editing speed. Also, be sure that your video card is up-to-date, from the chip or manufacturer's WEB site. Check Windows Direct-X as well for updates. See this FAQ.