When the project is big the backup is very slow.
I know that it’s not a real bug but It could improve the usefulness of the program if the prog could backup only the modified file.
In most cases that wouldn’t be desirable. This sort of backup system, where each backup is discrete and able to exist on its own, has an important place in the scheme of things, and doesn’t require a lot of complicated programming and user interface for retrieving backups from in the middle of the delta chain. That’s really the sort of thing for a dedicated backup system, not just some random writing program. For large projects you can disable automatic backups in the File/Back Up/ sub-menu, or you can experiment with turning off the .zip option in preferences, which will save a lot of time with larger projects. If you use Time Machine, then you’ve already got delta based backups, which makes turning off automatic backups for large projects a less risky decision.