When rdiff-backup is running at each end of the ssh link, then the rdiff-backup processes can compute rolling checksums and exchange those to determine which files have changed (and where in the file the changes are). This is the main advantage (and the whole point, really) of the rsync algorithm.

But when using sshfs, rdiff-backup sees only two "local" files. It will then compute these rolling checksums on both files itself, by reading in each file. Surely this causes the entire file to be fetched by sshfs? And surely this will (in general) be slower than with an rdiff-backup process at the other end which is sending only checksums, not the whole file...

About the only time I can figure that sshfs access might be faster is during an initial backup, when none of the files actually exist in the backup, and so there are no rolling checksums to compute and compare.