Performance is the place to start for tuning your site.
Moodle 2.0 does more on each page than Moodle 1.9, so you would expect it to be a bit slower.
Also, Moodle 2.0 has a lot of new code, that has not been optimised very much yet, whereas Moodle 1.9 had a lot of mature code that had been tweaked for speed, so Moodle 2.0 will get faster as people find the bottle-necks in the processing and improve them.