should get better results with --vfs-cache-mode full and maybe tweak --buffer-size and --vfs-read-ahead
it depends on what you have plex scanning. just small amount of data per file such as basic metadata
or large amounts to generate thumbnails, Perform extensive media analysis during maintenance and whatnot
and might get better results using native tools for smb mount, not rclone. which is what i do.
check out my howto guide
https://forum.rclone.org/t/how-to-access-smb-samba-with-rclone/42754