The Quick Answer:
No, your customers will not have slow download speeds.
Why your uploads are slow:
Typically when ISP's like the phone company sell DSL lines or Cable modem broadband connections they are selling "Asynchronous connections". That's fancy talk for "uploads are not the same speed as downloads" and basically means that even thought you have 1.5 or 3.0 Mbps download speed you may only have 256k or 512k upload speed.
Since most browsing is using the download side of things (pulling webpages from the net as opposed to uploading files) most people do not notice and the phone company saves money on upload bandwidth, which costs ISPs more.
Your customers downloads:
The DPD servers are connected to the Internet with 100Mbps connections in a carrier class data center, so we can handle many customers downloading at the same time at the full speed their internet connection can achieve. Download speeds for customers will only be limited by their download connection speed, not DPDs connection speed or your upload speed.
An Example:
File size: 500 MB
Time for you to upload @ 512k/s upload speed: 2 hours 36 minutes
Time for customer to download @ 3.0Mbps download speed: 32 minutes
This is one of the benefits of using a digital product delivery service like DPD- customers get their files faster and you only have to deal with the slow upload once when you set up your product- we handle the rest!