ten thousands users slow login

Hello,

we are using ubuntu 12.04 with samba/winbindd for user accounts from a windows domain.
There are more than 100.000 users. A login by ssh or tty is done in an acceptable time.

But a login with lightdm takes more than a minute to verify the username.

I think the problem is in the Lightdm API.
To get a LightDMUser you have to use:
LightDMUser * lightdm_user_list_get_user_by_name (LightDMUserList *user_list, const gchar *username);
and
LightDMUserList * lightdm_user_list_get_instance (void);

so you have to get a list of all 100.000 users to get one user. This takes more than a minute.

Is there a way to get a LightDMUser without a LightDMUserList?