I can confirm that WebDAV performance is terrible when mounted as external storage. And that is with hosted storage connected to a speedy Nextcloud provider. See this discussion on S3 speeds in Nextcloud using Wasabi; hopefully it will include some benchmarks soon.
Having hosted on Pi’s and other devices since the first Nextcloud release, I can tell you it is faster to simply host a full Nextcloud instance on your device. Accessing the exact same data over external storage or federation is always super slow and pops up a lot of error/conflict notifications.