He should have actually reported it right away but as I understand it chose to make a little experiment to see if anybody else will find it. Nobody did in a weeks time, but there is still lots of time until the code is ready for a release, so it doesn't necessarily mean a disaster.
Everybody should test before commiting but a single contributor cant test for all configurations. One have to rely on others to get all of the possible combinations tested. And look: it worked right away!
Performance regressions are especially hard to find since most test frameworks don't measure it.
Obviously kernel developers should use more performance testing frameworks such as phoromatic but maybe they are relying on the automatic kernel tracker at phoromatic.com to alert them of just those kind of problems.