Some applications create sparse files instead of pre-allocation such as qbittorrent. Qbittorrent first create a sparse file with a large size and fill them later. But rclone mount upload its sparse size to the remote once write back timeout is over, which is extremely inefficient.
What is your rclone version (output from rclone version)
Which OS you are using and how many bits (eg Windows 7, 64 bit)
Ubuntu 20.04
Which cloud storage system are you using? (eg Google Drive)
Google drive
The command you were trying to run (eg rclone copy /tmp remote:tmp)
not sure your use case but i prefer to download a torrent to local storage, not using rclone mount.
when the torrent has finished downloading, the torrent software runs a script using rclone move.