If you send out email through port 587 (submission), you can disable content filter (spamassassin, clamav, DKIM, disclaimer) completely by modify 'submission' entry in /etc/postfix/master.cf like below:
submission inet n - n - - smtpd
-o smtpd_enforce_tls=yes
-o smtpd_sasl_auth_enable=yes
-o smtpd_client_restrictions=permit_mynetworks,permit_sasl_authenticated,reject
-o content_filter=
What it does is simply appending line '-o content_filter=', then Postfix won't invoke any content filter for clients who send out email through port 587.
WARNING: DKIM signing is provided by Amavisd, so above modification means your outgoing mails won't be signed with a DKIM key.
To speed up Amavisd, here's some tips:
*) use a tmpfs to speed up disk I/O (it uses memory directly instead of hard disk): http://www.arschkrebs.de/postfix/amavisd_tmpfs.shtml
*) Set '$max_servers' to the number of your CPUs. And it MUST be the same as the number defined in /etc/postfix/main.cf. For example:
# In amavisd.conf:
$max_servers = 8;# In /etc/postfix/main.cf.
# NOTE: 'smtp-amavis' is the transport name defined in /etc/postfix/master.cf. We use
# [TRANSPORT]_destination_recipient_limit instead of modifying /etc/postfix/master.cf.
smtp-amavis_destination_recipient_limit = 8
*) Bypass one more check in amavisd.conf (in $policy_bank):
bypass_virus_checks_maps => [1],
bypass_spam_checks_maps => [1],
bypass_banned_checks_maps => [1],
bypass_header_checks_maps => [1],
*) Disable SNMP tracking if you don't use it at all. It's used by 'amavisd-nanny' and 'amavisd-agent'.
# In amavisd.conf
$enable_db = 0;
*) Retire an Amavisd child process after that many accepts. Default is 20, you can increase it on a busy server, for example, 40, or even lager.
# In amavisd.conf
$max_requests = 40;
Please do let us know whether it helps or not.