I have 2 productions servers both running open source 6.0.5. When sending mail from the web client the browser basically freezes for about 5-10 seconds. This only happens on 1 server (which happens to be substantially more powerful), load is only about .20. Every other function runs fine. Here's the relevant log:

Apr  9 11:21:56 mail postfix/smtpd[27583]: connect from mail.carischinc.com[]
Apr  9 11:22:04 mail postfix/smtpd[27583]: 881A29D41AB: client=mail.carischinc.com[]
So this particular email took 8 seconds to send. I immediately thought it was a DNS issue but everything looks good.