Comment 4 for bug 279654

I wanted to post to confirm the bug. I'm using Kubuntu 8.10 with KDE4 and running an NVidia graphics card supported by their proprietary drivers. 'kwin' consumes large amounts of CPU when you highlight and drag text in Firefox. When you highlight more than a line or two, this results it the system hanging for extended periods of time. Highlighting just one line still has a working text-drag effect (with very high CPU consumption).

On Windows and KDE3 with Firefox 3, the text-drag action results in the selected area being rendered in a transparent (and, depending on the quantity of text, scaled) form which follows the cursor around until released. The KDE4 version (when selecting small enough quantities of text to avoid hanging) seems to be visually similar to the KDE3 version.

You are correct when you say that it's related to desktop effects, as disabling them completely solves the problem. Furthermore, changing the Compositing Type from 'OpenGL' to 'XRender' also solves the problem, so it seems like it's a KDE4/OpenGL issue that may also involve (or be caused by) the NVidia proprietary driver.