I'm just back from an overnight trip where dial-up was my only option for getting on the web.
Several of the sites I wanted to visit were more than just slow, they were utterly unusable.
I couldn't even get logged in in a couple of places, let alone complete the tasks that I wanted to. One of those tasks would have involved spending money, but I couldn't get far enough into the site to do that.
I would highly recommend to every one who calls himself/herself a webmaster or to anyone whoever commissions a website: do not sign off on the project until you have personally tested the site on dial-up, and understand what kind of experience your dial-up users would have when trying to use the core functions on the site.
Up to a point, slow is understandable, but the site should not be impossible!