From Fedora Directory Server

Do I need to turn off access log to improve system perfomances

If your server generates several dozens of mega bytes of logs per hour, although the system is designed for minimal impact of processing and disk i/o's, depending of your system ressources, it may be recommended to disable access logging in some situations.

Turning on the access log buffering is expected to improve performances:

nsslapd-accesslog-logbuffering (Log Buffering)
http://www.redhat.com/docs/manuals/dir-server/cli/config.htm#pgfId-17719

When set to off, the server writes all access log entries directly to disk.

Entry DN: cn=config
Valid Values: on | off
Default Value: on
Syntax: DirectoryString
Example: nsslapd-accesslog-logbuffering: off

Example:

/opt/fedora-ds/shared/bin/ldapsearch -D "cn=directory manager" -w <some-pw> -s base -b cn=config cn=config nsslapd-accesslog-logbuffering
version: 1
dn: cn=config
nsslapd-accesslog-logbuffering: on

/opt/fedora-ds/shared/bin/ldapmodify -D "cn=directory manager" -w <some-pw>
dn: cn=config
changetype: modify
replace: nsslapd-accesslog-logbuffering
nsslapd-accesslog-logbuffering: off

/opt/fedora-ds/shared/bin/ldapsearch -D "cn=directory manager" -w <some-pw> -s base -b cn=config cn=config nsslapd-accesslog-logbuffering
version: 1
dn: cn=config
nsslapd-accesslog-logbuffering: off