[ocis] Improve user listing speed in `Admin Settings` > `Users`

## Description
relates to https://github.com/owncloud/enterprise/issues/6597 and https://github.com/owncloud/web/issues/10821

## Userstories
> As an admin I want a quickly and reliably responding usermanagement so that I have control over all user accounts.
> As an admin I want to select all pupils to increase their quota.
> As an admin I want to select all pupils from the senior classes to put them into the group "Coratia Field Trip".
> As an admin I want the feeling that the user management ist reliable so that I feel in control of who can access the instance.

## Steps to reproduce

1. login as Admin on an instance with ~500 users eg. on INT 0387 as user INT-o.dewald (login detailshttps://github.com/owncloud/enterprise/issues/6597)
2. Go to Admin Settings > Users
3. Filter eg. for Role `Schüler/-in` or search for the letter `e`
4. Very slow response: It takes ca. 15 seconds until a result is shown

## Expected behavior
Results should show up quickly, eg. < 3 seconds

## Actual behavior
Very slow response: It takes ca. 15 seconds until a result is shown

## Solution Scope for optimisazion