If you have a lots or very big LDAP groups in your mapping configuration it might take a while to cache all of them. If that’s your issue, http://platform.xwiki.org/xwiki/bin/view/AdminGuide/Performances#HDisableLDAPsubgroupssearch might help.
As for user created without the object and then modified it’s indeed not very nice and mostly caused by laziness. Needs to improve https://github.com/xwiki-contrib/ldap/blob/master/ldap-authenticator/src/main/java/org/xwiki/contrib/ldap/XWikiLDAPUtils.java#L1276. But the time between the two saves is supposed to be very small.