QtWebKit fails to render web fonts on OS X 10.9 Mavericks

QtWebkit is extremely slow and it fails to draw font-face'd fonts. The text appears to have empty regions where letters are supposed to be, but not drawn. I have received reports from ALL my users about these two issues and it renders my Qt app inoperable on Mavericks. I have also upgraded, and I am having the same issues as well. Check the attachment for demonstration of the issue.

I have also tried to update to the last 5.1.2 beta and the same problem also exists in the beta, too.