If we pre-generate the files for the build instead of building them whilst running PHPUnit, we can see massive performance increases.
If the files don't exist, they should still be generated so the existing behaviour is maintained though!
The second phpunit call could be run without xdebug because we call it with the --no-coverage option.
Yep, but it's the generation of the files (done in the first phpunit run) that takes the time (~10m with xdebug, ~2.5m without)
#1135: update travis configuration