The Cairo back end that Firefox is now using is optimized for X servers implementing the XRender extension. It will work with out this extension, but performance suffers, particularly when running over a network. The details of this problem are available in freedesktop.org bug number 11529. The Xvnc server does not implement XRender, which causes severe performance degradation, especially if Firefox and Xvnc are not running on the same machine. Xvnc is a popular package, but does not seem to be actively developed, at least on the server side. The root of the problem seems to be that in the absence of XRender, cairo must use a sequence of XGetImage/XPutImage calls for every image it draws on the screen. These operations magnify the effect of network latency. I believe this could be worked around by rendering the page to a pixmap and sending the finished page to the X server instead of constructing it there piece by piece. This would obviously not be a win for the general case, so it would need to either be autodeteced or be a preference. I do not know if the code lends itself to that or not.

May be related to 386440 & 299727

I don't know what changed in Firefox 3 Beta 4, but rendering to a VNC X server is MUCH faster.

As the reporter says, the problem seems to be no longer there. Marking as FIXED until someone proves otherwise.

No patch / bug given as the code fix.