If somehow Internet Explorer is slow, then clicking links from Office will be slow as well, even if you don't use Internet Explorer as your default browser. So: check if Internet Explorer is still running fine.
Though you seem confident that DDE is to blame, Office has a surprisingly odd feature: it first uses an Internet Explorer component to see if the URL one clicks is valid. It does not identify itself as Internet Explorer; in the access logs one might see:
User Agent: Microsoft Office Existence Discovery
After that, it hands the resulting URL to the default browser. That is:
If the hidden call to the URL yields some redirect, then the default browser is not even given the original URL, but the redirected URL.
If the web site for some reason blocks the User Agent "Microsoft Office Existence Discovery", or if your Internet Explorer settings somehow prevents proper access to the site, then the link might seem dead while in fact using a normal browser it would work fine.
Ever wondered why your browser keeps redirecting you to some login page when clicking links from Office? Right: if Internet Explorer is not authenticated at the web site (especially true when it's not your default browser), then some sites might respond with a redirect to a login page, making Office forget about the URL you actually clicked...
Some more details about this
funny annoying "Microsoft Office Protocol Discovery" at Microsoft's Description of the Microsoft Office Existence Discovery Protocol blog post:
When opening documents from a URL location in Microsoft Office 2007, the Office library can make an HTTP HEAD request to the web server for the opening URL. This request is sent with a User-Agent set to"Microsoft Office Existence Discovery". This call is new to Office 2007.
The purpose of the HEAD request is to check that the content exists at the URL location as a document, and not simply as a tempoary resource streamed down for a read-only session. The call will also attempt to obtain the last modified time of the content as returned by the web server in the HEAD response.
This call occurs on all URL open attempts, even if editing is not requested per se. As a result it is possible that the extra web call (made from the process space of the Office application in its network session and not the web browser in a separate session) can cause some users to see extra prompts to authenticate (401) or loss of session state and an unnecessary redirection (302) to a login page or other feedback form. This is expected behavior.
It seems this can be disabled using the registry; see my answer at MS Word validating links after click.