Sites like Google and Yahoo are connect almost directly to the backbone of the Internet. Sites like JD and RS are usually in data farms, which are usually connected with a pretty fast connection to the backbone, but not nearly as fast as main sites. So there's usually a speed difference there.
But thinking about it, I think it'd be more likely to be a problem with your ISP's DNS server, or their connection to the server that resolves domains. Basically, when you type in a URL, that data shoots out to your ISP, who then sends a request to a domain controller server. That server gets the URL, and returns the IP address associated with that URL. That IP directs your provider where to look for the site. For something like Google, that IP would be in the cache on your provider's server because it's so frequently accessed. For JD, that would be accessed much less frequently, so it'd be flushed from the cache and need to be looked up every time.
Your personal PC should store most of the recent IPs from sites you've visited, which should allow it to bypass this process. But you can fiddle with a browser not to store IPs like that and rely on your ISP.
But really... DLS sucks balls. You should have gotten a cable modem. Then you wouldn't have any problems.