If you have performance problems with backup of a large PostgreSQL database, then you may ask about incremental or differential backup.
An incremental backup is a backup of all files that have changed since the last full backup. In order to restore, you must restore the full backup and then each set of incremental changes.
A differential backup is a backup of all individual changes since the last full backup. Again, restoration requires you to restore the full backup and then apply any changes since then.
To perform a differential physical backup, you can use
rsync to compare the existing files against the previous full backup, and then overwrite just changed data blocks. It's a bad plan ...