Running Roundcube version 1.4.9 which I built from source, though I have had this issue for a while with the stock Ubuntu version 1.4.3 (and older versions too, though I can't say precisely when this started)
I've not yet been able to isolate precisely why this is happening or where in the code, but some emails consistently have this issue. Click to load the message and it takes 5-6 seconds to completely load. Some emails load fine consistently. Some take 5-6 seconds to load consistently. The same senders tend to have issues, but not always. This is not an IMAP or SMTP or DNS issue from what I can tell - it has something to do with generating the stylesheet in the task=utils or action=modcss call, I believe. Almost all of this time (~5.2 out of 5.4 seconds)is spent in the following call:
?_task=utils&_action=modcss&_u=tmp-5f419b7f3c624a8a0ddc3a6212c91d5f.css&_c=message-htmlpart1
Note that if I go to the above link directly on my webmail server all it's doing is generating the attached stylesheet. So why does this take 5+ seconds?!?!
Attaching the network timing info/waterfall from Chrome. Also attaching a sample email that causes issues. Note headers have been scrubbed of personal username/hostname with $USER/$MAILHOST but message is otherwise intact.
Side note: I went and disabled all plugins thinking perhaps one of them was triggering this issue but it made no difference in performance.