It's not a tiny bit at all. For checksum, the whole file need to be read beforehand, the metadata is just a quick lookup. It's order of magnitude slower and eats I/O too.

Checksums are slow on HDDs and using SSDs for backups is a bad idea, particularly for cold backups, due to data longevity and the tendency for SSDs to experience sudden total failures instead of gradual degradation.

The -c makes it do more reads, though, not unnecessary writes

Degradation won't get worse except when a file changed without metadata having been modified, but then that's exactly what you want

Yes, but it's orthogonal to my comment.

Because changing the defaults of command-line tools is generally a shitty thing to do.

And the speed difference is only tiny for tiny files.