In my experience with DreamHost, mail generated from a form in this manner can have greatly varying lengths of delivery times. I have had some arrive almost instantly while others takes "hours", though most arrives within a few minutes most of the time.
I've always assumed it probably has to do with being queued at sending the mail server. It could be it is just "taking it's turn in line" with all the other outgoing mail generated that way, and is possibly throttled to help manage load on the mail server and to mitigate some ISP's perception of it being "spam" based on sheer volume.
Of course, I could be way off base on that (I have no personal knowledge of how DH manages such things), but it makes a certain amount of sense. I suggest monitoring it for a bit and see what your "general" experience is; it may not be much of an issue if your experience mirror mine, or you may want to investigate further if all, or much, of such mail is delayed by that much.