I agree with you that this issue seems to have been thoroughly ignored by the release engineers, who pushed the 2.6 driver through despite its downfalls. (apparently to gain support for some newer chipsets, which could've been simply backported by adding PCI ID's to the 2.4 driver)
Compositing and 3D performance are horrid and even non-composited 2D is slow when EXA is used. Apparently, if you're lucky you can switch to UXA, but it is non-functional on my systems. (G35: no modes found & i915: DRI disabled)
The issue can be improved (but not fixed) by using a backport of the version 2.4 intel driver available from the "http://ppa.launchpad.net/siretart/ppa/ubuntu" PPA of siretart. This provides minor performance enhancements but is still slower than intrepid.