Depending on the DSL service, it may not really be all that fast, especially when uploading. There are a lot of badly tuned/overcommitted DSL connections out there.
If the users are complaining about send times, they may be getting thruput only about 50kbps, so any amount of data, esp. graphics will be noticably slow. HTML messages can be a meg easily with embedded graphics. Try text.
When are they experiencing delays?
Is it consistent or just when their kids are FTPing MP3's from another machine? (grin).
Jock