[Bug]: Task sync very slow with a lot of Lists with a lot of Tasks

⚠️ This issue respects the following points: ⚠️

• This is a bug, not a question or a configuration/webserver/proxy issue.
• This issue is not already reported on Github OR Nextcloud Community Forum (I've searched it).
• Nextcloud Server is up to date. See Maintenance and Release Schedule for supported versions.
• I agree to follow Nextcloud's Code of Conduct.

Bug description

The sync is slow and it causes the Task.org app synced via Davx5 to be very slow and buggy, here is the Issue i created on Tasks.org: https://github.com/tasks/tasks/issues/2856

Steps to reproduce

1. Have a lot of lists with a lot of tasks
2. Sync them with Davx5 into the Tasks.org app

Expected behavior

The sync should be fast

Installation method

Community Docker image

Nextcloud Server version

28

Operating system

None

PHP engine version

PHP 8.2

Web server

None

Database engine version

None

Is this bug present after an update or on a fresh install?

None

Are you using the Nextcloud Server Encryption module?

None

What user-backends are you using?

• Default user-backend (database)
• LDAP/ Active Directory
• SSO - SAML
• Other

Configuration report

No response

List of activated Apps

No response

Nextcloud Signing status

No response

Nextcloud Logs

No response

Additional info

I have 21 Lists with 547 open tasks (sum of all lists) and i don't know how much closed ones, i would say 3000+ because i use this app combo since a few years now. It was always fast and i don't know when it became slow but last summer (2023) it was still fast, so it should be sometime in fall - winter 2023.

I don't know what the problem is and i don't know how to find out where the problem is, so any ideas how to find out are highly appreciated.

No source file, test, logs, or server component is named. Start by reproducing the DAV sync with 21 lists and roughly 3,000 tasks, then collect the missing Nextcloud logs and configuration details to identify where the slowdown occurs. Done means the sync is measurably faster for this dataset.