Reference Variable Performance Issue

[AdriaM]

We have a Performance issue with a reference variable on sys_user table. We need to restrict the selectable users to the users who belongs to a set of groups of certain type (14000 users).

If we use a reference variable and develope an advanced qualifier that returns an 14000 sys_ids length array, Service Portal form loads quickly but it takes over 10 seconds to return filtered users.

If we use a lookup select box on sys_user_grmember table it takes over 30 seconds to load Service Portal form but filtered users are returned quickly.

We also need a List collector and we only can use the advanced reference qualifier on sys_user table that is also slow.

We've tried "performance hacks: reference variable" articles and they're not working also.

Has anybody faced the same issue and have found a good solution?

Is there a way to filter a Reference Variable without having to return 14000 sys_ids?

Is there a way to use the "Lookup select Box" without loading all users at first, only load users when are typed by the user?

Is there a way to use a "List collector" but defining the available items like in a "Lookup Select Box"?

Is there a way to use a "Lookup Multiple Choice" like a list collector instead of generate radio buttons?

Thank you,