I've been hoping this would improve as versions went on, but it hasn't. Whenever I do anything with 2.7 that involves writing files or info (new post, new comment, saving settings, loading media) it takes an average of thirty seconds to process. I've timed it.
I disabled all plugins (and functions.php stuff) just to make sure none of them were interfering, and it had no effect on the slow-down.
I am running 2.7 on the same server as several installations of 2.6.3, and none of them have this problem. It's an Apache 2.2.9 server running PHP 5.2.6 and MySQL 5.0.67.