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.

PHP 5 without code coverage

$ php56 vendor/bin/phpunit -v
Runtime:       PHP 5.6.15
Time: 4.78 seconds, Memory: 40.25Mb

PHP 7 without code coverage

$ php70 vendor/bin/phpunit -v
Runtime:       PHP 7.0.0RC6
Time: 3.37 seconds, Memory: 22.00Mb

So, PHP 7 is faster, gain 30% of time and 45% of memory.

PHP 5 with code coverage

$ php56 vendor/bin/phpunit -v
Runtime:       PHP 5.6.15 with Xdebug 2.4.0beta1
Time: 1.89 minutes, Memory: 90.50Mb

PHP 7 with code coverage using XDebug

$ php70 vendor/bin/phpunit -v
Runtime:       PHP 7.0.0RC6 with Xdebug 2.4.0beta1
Time: 39.41 seconds, Memory: 52.00Mb

Again PHP 7 is really faster (65% of time, 43% of memory)

PHP 7 with code coverage using phpdbg

$ php70-phpdbg -qrr vendor/bin/phpunit -v 
Runtime:       PHPDBG 7.0.0RC6
Time: 13.07 seconds, Memory: 92.00Mb

Terribly faster :) 66% of time saved compared to Xdebug, and 89% compared to PHP 5

I notice a lot of developers are not aware of the latest solution, what a pity! I hope this will encourage you to test it.