I'm not sure if it's because of locking resources or threading done bad, but in my experience rekonq suffers from becoming very unresponsive when it's loading pages. KDE has otherwise taken the ideal of separation of logic and presentation well to heart, so I can't imagine it being single-threaded.
It's a good fallback browser and its feature-set is actually impressive (though it could use greasemonkey support), and its (reference Qt) webkit engine only rarely fails to render pages completely. Graphical oddities such as oddly-sized elements are very common, mind.
But the performance that Chromium offers is just... too compelling.