Version:           3.5.6 (using KDE 3.5.6, Kubuntu (feisty) 4:3.5.6-0ubuntu14)
Compiler:          Target: i486-linux-gnu
OS:                Linux (i686) release 2.6.20-16-generic

When reading mailing list digests, KMail takes a very long time to render the messages. When a digest email is selected in the Inbox for instance, KMail renders the "fancy" view, where the contents are listed on top in plain text, and each encapsulated message is rendered with an outline that looks like a standard message.

Whenever such a digest message is selected, KMail takes anywhere from 10 to 30 seconds to render the message; and during this time the entire KMail gui is unresponsive. It also affects other running applications: for instance, switching desktops and other activities are slowed while KMail is performing this render; though KMail is the most-affected application, and it is totally unresponsive until it finishes. It typically takes about 15 seconds to render the message; and it happens every time the digest is selected for view: it is not a one-time only event.

For a concrete example, the Tapestry Users mailing list digest exhibits this symptom on my machine with 100% consistency, and it takes quite some time for KMail to render the digests. That list can be subscribed-to here:

http://tapestry.apache.org/mail-lists.html

And select the "Tapestry User List".