As mentioned in a sibling comment, linux.die.net's manpage is outdated here (covering rsync 3.0.6). Current versions of rsync (>= 3.2.0) autonegotiate checksum type between several different options (including a few variants of xxhash, md5, and md4) unless overridden by the user, or one side is pre-3.2.0 (in which case you get the old behavior with MD5 or MD4 depending on protocol version). The pre-transfer and transfer checksums also don't necessarily have to be the same (I don't particularly care to hunt down the default priority lists right now, so I'm not sure if they are the same or not by default).
I was surprised to learn recently that sha1 is typically about 10% faster than md5. I have some coworkers who I need to remind of this, since they’re still using md5 for caching.
Newer versions (≥3.2?) support xxHash and xxHash3:
* https://github.com/WayneD/rsync/blob/master/checksum.c
* https://github.com/Cyan4973/xxHash
* https://news.ycombinator.com/item?id=19402602 (2019 XXH3 discussion)