Good news: We have addressed recent SSH performance degradation by replacing linear parsing of the authorized_keys file with a custom AuthorizedKeys command.

This is what GitHub and GitLab have been doing for years, and we have grown to a size where this has become necessary for us as well.

The cause for the degradation was still abusive patterns, as connections without valid key take more resources (scanning the file to the end) than legitimate users (scanning is stopped after match).