the biggest weakness of rsync is that it's single-threaded. Single-thread-copy with millions of small files is painfully slow, Rsync is as good as it can be but you just need more threads.

I think that's what a sibling is getting into with the "better to tar files and send them over ssh in some cases" thing. And yes, you can hack it in after-the-fact with xargs/etc but it's clunky compared to just having native multithreading like rclone/etc.