Nextxcloud and Synology: Upload via webdav very slow


using nextcloud on my webspace and want to sync Files via “Cloud Sync” from my Synology NAS. Connecting via WebDav works finde - the big problem: the upload speed is very slow (~150-200 kbit/s). When using Cloud Sync with my Dropbox Account the speed is much higher.

My Internet has more or less 25Mbit/s Download and 5Mbit/s Upload Speed.
The Synology (DS218+) is connected via 1000 lan to the router.
Synchronisation of files in the local network works perfect.

Does anybody have a similar problem or has an idea?

Kind regards!