bridgman: Yeah, there was a new Intel driver which relied on the downloaded data to overwrite old data in newly allocated memory buffers. Nothing wrong with that AFAIK (presumably it was done as a performance optimization) but it interacted badly with the no-backfill patch, allowing users to see old screen info which potentially could have been from another user's session.
The problem was flagged as a security issue and removing the patch was the most expedient way to close it. The result was that any driver running XAA acceleration became very slow when performing certain operations. Felix implemented the backclear patch which performed the same function as no-backfill but also added code to clear out the destination buffer, eliminating the security hole on Intel GPUs while still providing good performance on XAA drivers.
When the 10.6 driver comes out, check the release notes to see if the new 2D acceleration code is enabled by default. If so, give that driver a try and it should address some of the problems you are seeing.