I have PIX515 (IOS 6.22) with mail server in dmz.
No other network devices in dmz.
Clients from inside access dmz via dymanic NAT.
No static NAT and ACLs are applied.
POP3 and any other types of connections from inside to dmz mail server are fast, but SMTP slow ( mail server responses to 'Telnet <ip_addr> 25' over 5-10 sec). No errors are occured on both ethernet interfaces.
No problems with client performance and mail server when it moves to inside