Drupal's issue queue currently has a running thread regarding Drupal 7's potential new database layer based around PDO (Database Layer: The Next Generation).
I have spent some time looking at this system (starting when it was just a sample "Core" in chx's private SVN repos through to the new patch linked to above) with a view to comparing this new abstraction layer with that found in Drupal 6. The point of this exercise wasn't to see if the new system was a roaring performance gain but more to check that it hadn't potentially gone the other way.
So, on to the results of the patch to date. This was tested by creating a fresh Drupal 6 install and used the devel module to populate users, taxonomy and content. A mysqldump snapshot was taken before the tests so as to provide a rollback position for furture tests.
Next, a CVS HEAD checkout was made and the patch at #17 was applied.
The front page was then profile numerous times to find some basic averages of Drupal 6 verses HEAD+patch.
The average page generation time for Drupal 6 drifted between 535ms to 590ms.
The average page generation time for HEAD+patch drifted between 525ms to 575ms.
As for the general code path review for these can be seen here:-