Remi Collet has a quick review of some of the performance results from running the Composer PHPUnit tests on PHP 5 versus PHP 7.
As already said numerous time, PHP 7 is faster than PHP 5. Since PHPUnit 4.8 you can choose between XDebug and phpdbg as driver to retrieve code coverage data, see PHPUnit 4.8: Code Coverage Support.
Here is some benchmark results. All the tests are run using PHPUnit 5.0.8, PHP 5.6.15 as SCL or PHP 7.0.0RC6 as SCL and XDebug 2.4.0beta1 (freshly released, with some additional patches) for the composer test suite.
He shows the results in execution time and memory used for PHP 5 versus PHP 7 versions, both with and without code coverage being generated. He also includes two examples of running the tests with PHP 7, once using the XDebug debugger and one using phpdbg.