The performance patches from http://moodle.org/mod/forum/discuss.php?d=79455 need to be examined, tested.
1) Examine new code in various environments and file/fix bugs here as necessary
2) Test common operations such as logging in, listing courses, searching courses, moving around, assigning roles, using modules, switch user, login as on:
a) small sites
b) large sites (25,000 students / 10,000 courses / 100,000 enrolments)
and measure page generation times and database load for current HEAD and new code
3) Along the way keep fixing any divergences in functionality by posting them here.
4) If after all that, if the new code is better overall by, say, 20% or more then let's add it to HEAD ASAP.
5) Continue testing in HEAD.
Make sure you have a look at the testing and dev hints mentioned in the forum thread above.