LDAPNOINIT skips the reading of the ldaprc override file.
The ldaprc file is an optional configuration file which would be used to override system-wide defaults.
It can contain information just like the slapd.conf file. For example, a keyword followed by a value. the ldaprc file is an override to the slapd.conf since as the ldap protocol states, the slapd.conf file is read first and then the ldaprc file (if one exists) is read second.
By placing the LDAPNOINIT environment variable in the slapd.env file, you tell LDAP that during initialization, don't even check for the existence of the ldaprc file since all of your configurations are in the slapd.conf file. Since LDAP doesn't check for the ldaprc file, you save time during initialization.