Uploading from android app is slow

When I upload files from android app to a external local storage on my nextcloud server, the upload speed is only 1~2MB/s.
When using web interface or a 3rd-party webDAV client like ES File Explorer to upload to the same location, it speeds up to 8~10MB/s instead.

But I must use the app because only nextcloud app provides documentprovider storage so I can store backups of other apps directly into my server. Why it’s so slow?

Android app version: 3.23.1(F-droid)
Server: nextcloud 25.0.1 installed by using turnkey nextcloud lxc 17.1-1 (apache2+mariaDB)