Sebastian Bergmann quickly points out a patch that Derick Rethans committed to the XDebug software to make things a bit more usable:

Derick recently committed a patch that improves the performance of code coverage data collection in Xdebug. [...] This makes PHPUnit's code coverage functionality so much more practical to use.

Two examples of it in action are included - one for XDebug 2.0.0 and the other for the development version, XDebug 2.0.1-dev.