Remote/external ftp storage very slow

we run the latest nextcloud, with external stoarge mounted via ftp

the connection to the ftp server is testet to be some 100 mbit
a curlftps mount in an linux runs seamingly fast

why is that a nextcloud showing some random directory (with several subfolders and plenty of GB on Data) takes Ages … at least several Minutes - or fails completly

what does the script is doing/searching for?
it looks like the script is running and wants to validate all the folders and filesizes (which it ould do in background)

this renders the remote/external file feature unusable

any tips to circumvent this - or the point / structure which maintains the ftp remote stuff would be awesome, thx