Hi
On tiny nas, owncloud and other similar web apps use too many resources and yes webdav is better because webdav client environment uses only resources on the user side. So only nas resources on the web serveur is used to transfer/update files.
Otherwise the default settings are not optimized for a web use. You have to increase resources having to be used by the web server (main setup menu, web server tab)
This must be done for owncloud, web-explorer and webdav.
About web-explorer about file manager, you have to set some settings to allow what user account can do that (by default read-only)