Diff two directories in Linux

For a couple of Linux/Unix systems I run daily backup over rsync.
The filesets are nearly static, so the the backup duration is nearly identical each day. Between 22 and 30 minutes for 21-systems. But there are also days, with a longer backup durations.

With the following diff and rsync commands I was able to identify the differences between the backup directories/states, which files are get changed over the time.

rsync -rv --size-only --dry-run /source/ /dest/ > diff.out

diff -qr /source /dest/

The conclusion of this tiny backup research was, I was able to exclude more files from backup, which are changing each day, but are not relevant for recovery.