I just remembered we have discussed this before in the forum: Ls slow on large directories (500~ files) (Hubic)
Number of checkers makes no difference as it is one per directory
I made this issue about it: swift: speed up directory listings with lots of big files · Issue #1791 · rclone/rclone · GitHub
rclone could defer reading the size until it is actually used. That would work too I expect.
You can try the --ignore-size
flag - I'm not sure that will help though.
I don't think swift list reads the size does it?