docs(user): add slow WebDAV performance to Known Problems section

Performance degradation when using WebDAV with a regular password instead
of an application password was only mentioned as a general note at the top
of the page. Users troubleshooting slow transfers are unlikely to find it
there. Added a dedicated Problem/Solution entry in the Known Problems section
with keywords (slow, times out) to improve discoverability.

Fixes #13920

Signed-off-by: skjnldsv <skjnldsv@protonmail.com>

Author: skjnldsv

user_manual/files/access_webdav.rst

@@ -370,6 +370,26 @@ in a WebDAV client, use the share token as username and the (optional) share pas
 Known problems
 --------------
 
+Problem
+^^^^^^^
+
+WebDAV access is very slow or times out.
+
+Solution
+^^^^^^^^
+
+When using a third-party WebDAV client (including your operating system's built-in
+client), you should authenticate with an **application password** rather than your
+regular account password. Using a regular password carries a significant performance
+penalty because Nextcloud must perform additional work to verify it on every request.
+Using an application password removes this overhead and can improve transfer speeds
+substantially.
+
+To create an application password, log in to the Nextcloud web interface, click your
+avatar, choose **Personal settings**, then open the **Security** section in the left
+sidebar and scroll to the bottom. Create an app password there and use it in your
+WebDAV client instead of your main password.
+
 Problem
 ^^^^^^^
 Windows does not connect using HTTPS.