/usr/bin/top always asks for all usernames on userid->username mapping. For performance reason. See: root/contrib/top/username.c (L166)
But if you have an LDAP infrastructure with thousands of users than the request for all users will end up in:
nslcd: [99e99b] <passwd(all)> ldap_result() failed: Size limit exceeded
Therefor it would be better if the uid->username mapping is only for used uid's.