The only time this might happen is if the ADSL network becomes heavly utilized at the same time that you are transferring data. Your ISP has no control over how Verizon sets up its data network. The total number of users your ISP attaches to its ADSL network will affect the performance for each of those users. It is up to your ISP to monitor its network to ensure it maintains a good balance of users and good bandwidth utiliziation across each of its circuits in order to ensure that their customers retain respectable data rates. ALL THE TIME with no chance of slow-down then a service like ADSL which employs shared bandwidth is not for you.
Frame relay provides guaranteed bandwidth with much higher CIRs.
Keep in mind that it is also a more expensive service, usually recommended for business clients with multiple users. CIR is the reason your ISP is able to sell such a high bandwidth service for such an incredibly low price. All in all, ADSL will provide very high burstable bandwidth at an incredibly economical price. Ultimate upload and download speeds depend upon how far your location is from the Verizon Central Office, the condition of your telephone line, the gauge of the copper that comprises your telephone line, etc. Rates quoted on the ADSL pricing page are maximums only and are not guaranteed.