I can't speak for twenty years ago, but rsync always gives me line speed for server-to-server copies in my workshop.
The CPU use might be slightly higher. I don't think it's a problem anymore.
It can be slow if you're transferring a large number of files since it does an index before it starts.
Since it's release in the year 2008, rsync 3.0.0, the recursive algorithm used is now an incremental scan that uses much less memory than before and begins the transfer after the scanning of the first few directories have been completed.
OTOH there where issues with scp having a fixed tcp window size of 4k (not sure if this is still the case) which would slow down copies on networks with some latency.
sftp have none of these issues. i prefer sftp for most tasks.