Please list the specs for your PC - CPU, GPU, disk space, operating system, etc. Video editing is intensive. Kill all other programs while editing.
It is possible that the VP cache has become corrupt. Though it may initially slow project loading, try clearing that cache. TOOLS menu|OPTIONS|DISK.
Have you defragged the drive lately (provided it's not an SSD)? Windows Control Panel|ADMINISTRATIVE TASKS|DEFRAG
Have you cleared windows temp files? In Windows (File) Explorer, right-click the C: drive and click CLEAN UP.
Under the VP FILE menu, back up project files to a folder, than save the project. SAVE AS regularly, under incremental names, so the earlier saves are not overwritten.