Disable observers for Mage_Log. Module can not be disabled completely because other modules rely on it. Don't do this if you need the product compare feature.

performance-mage_log.xml

<?xml version="1.0" encoding="UTF-8"?>
<!--
/**
 * Disable observers for Mage_Log module.
 */
-->
<config>
    <frontend>
        <events>
            <!-- Disable Mage_Log -->
            <controller_action_predispatch>
                <observers>
                    <log>
                        <type>disabled</type>
                    </log>
                </observers>
            </controller_action_predispatch>
            <controller_action_postdispatch>
                <observers>
                    <log>
                        <type>disabled</type>
                    </log>
                </observers>
            </controller_action_postdispatch>
            <customer_login>
                <observers>
                    <log>
                        <type>disabled</type>
                    </log>
                </observers>
            </customer_login>
            <customer_logout>
                <observers>
                    <log>
                        <type>disabled</type>
                    </log>
                </observers>
            </customer_logout>
            <sales_quote_save_after>
                <observers>
                    <log>
                        <type>disabled</type>
                    </log>
                </observers>
            </sales_quote_save_after>
            <checkout_quote_destroy>
                <observers>
                    <log>
                        <type>disabled</type>
                    </log>
                </observers>
            </checkout_quote_destroy>
            <!-- /Disable Mage_Log -->
        </events>
    </frontend>
</config>