synopsis: Using emacs 26.1 with tramp 2.3.3.26.1, renaming with dired turns into copy, which is much slower. This is most noticeable when changing directory names.
The two setups I am using are:
1. GNU Emacs 25.3.1 (x86_64-pc-linux-gnu, X toolkit, Xaw scroll bars) of 2018-05-24
2.2.13.25.2
2. GNU Emacs 26.1 (build 1, x86_64-apple-darwin17.7.0, GTK+ Version 2.24.32) of 2018-12-12
tramp 2.3.3.26.1
I have not tried rolling back to emacs 25.3 on the mac yet.
The expected behavior, which I see locally on setup #2 and in all cases on setup #1, is that a rename of a directory, done through dired (or wdired) does a 'mv' command, which is fast.
On setup #2, accessing a remote directory, when I rename a subdirectory, it takes a long time because the move turns into a copy command, which takes a long time (for large directories). This is true whether I use 'scp', 'ssh', or '-' (which is scp) as the connection method.
I looked into the dired and wdired code between the two versions of emacs and found nothing substantially different between the versions.
Also, possibly related, but not fully vetted, I find that a command-line rsync is much faster than copying directories using tramp (again, with method scp or ssh).
Please let me know if I've left out any salient details.
Peter