I've got an HTC Advantage X7501 running WinMO 6. The built-in e-mail program seems very slow retrieving messages from Comcast, on the order of a minute to see the connection, log-on, etc. The connection is over WiFi both at home and work.
As a comparison, a trial version of Pocket On Schedule does this in a manner of seconds. A trial of Flexmail is as slow as the onboard program.