OneDrive has Files On-Demand feature, and sometimes this feature can cause slow OneDrive download speeds. This is a quite useful feature since it allows you to download only files that you want to edit.
OneDrive supports rclone cleanup which causes rclone to look through every file under the path supplied and delete all version but the current version. Because this involves traversing all the files, then querying each file for versions it can be quite slow. Rclone does --checkers tests in parallel.