It was probably already mentioned earlier, but I noticed this only recently => somebody else might have missed it as well.

My syncs are now a lot faster (using new options "--compress-choice=zstd --compress-level=3 --checksum-choice=xxh3"). Output on Gentoo:

  # rsync
  rsync  version 3.2.3  protocol version 31
  Copyright (C) 1996-2020 by Andrew Tridgell, Wayne Davison, and others.
  Web site: https://rsync.samba.org/
  Capabilities:
      64-bit files, 64-bit inums, 64-bit timestamps, 64-bit   long ints,
    socketpairs, hardlinks, hardlink-specials, symlinks, IPv6, atimes,
    batchfiles, inplace, append, ACLs, xattrs, optional protect-args, iconv,
    symtimes, prealloc, stop-at, no crtimes
  Optimizations:
      SIMD, asm, openssl-crypto
  Checksum list:
      xxh128 xxh3 xxh64 (xxhash) md5 md4 none
  Compress list:
      zstd lz4 zlibx zlib none

Synced files up to a size of 400GiB => nothing was hanging (it could happen in the past, for misterious reasons) and CPU consumption was minimal.

Really happy about these improvements.