If you were willing to sacrifice best practices for performance, one obvious option is to accept only hmac-md5, which is very fast and still somewhat secure. I

If you were willing to sacrifice best practices for performance, one obvious option is to accept only hmac-md5, which is very fast and still somewhat secure. If the systems involved lack "AES-NI" native CPU opcodes, then you might revert to chacha20-poly1305, which is supposedly faster on CPUs that lack acceleration. This also overrides the hmac-md5 above, as it is an AEAD. If you needed to go faster still, then you could instead choose ARCFOUR (RC4) as you say, but this has been removed from the latest versions of OpenSSH because it is not safe. The worst of the above configurations are still not as bad as classic FTP.