Speed up data access to external storage (NAS via sftp)


I have a nc 18.0.7 instance and an external storage (NAS) connected via internet with sftp.
The NAS has 6 TB of files.

If I now access from nextcloud to this external folders, I must wait 15 seconds .
If I connect direct to the NAS with e.g. filezilla, I see the folder and content immediately.

I don’t know what happens in those 15 seconds?
How can I speed up data access?

Thank you very much! Michael