Faster phpunit-bootstraping for single Unit-Tests

Question

My team and I started on Drupal some months ago. As we began creating our own modules, I also wanted to test them, but was very annoyed by the Performance of the tests - or better: of the Performance of core/tests/bootstrap.php:

• It takes about 35 seconds after entering the phpunit-command until the tests have passed.
• The Unit-Tests themselves in my modules take 1-2 Seconds to pass.
• All the other time is spent in building up phpunit, especially in core/tests/bootstrap.php#drupal_phpunit_populate_class_loader()

The reason: Although I only want to test my module, bootstrap.php scans every path in all the modules, profiles and themes in core and contrib for populating $GLOBALS['namespaces']. IMHO, 30seconds of bootstraping for any tests prohibits fast and often testing and therefor makes test-driven development impossible.

I found that I can significantly improve the performance by calling phpunit with the UnitTest-Folder as argument (phpunit modules/custom/modulename/tests/src/Unit) and changing $paths in drupal_phpunit_contrib_extension_directory_roots():

  $paths = [
//    $root . '/core/modules',
//    $root . '/core/profiles',
//    $root . '/core/themes',
//    $root . '/modules',
//    $root . '/profiles',
//    $root . '/themes',
      $root . '/modules/custom',
      $root . '/themes/custom'

With this change, bootstraping the tests takes about 1-2 seconds, which would be accetable for me. But it works only for UnitTest, Kernel- and Browser-Tests run no more.

So my questions:

1. Is bootstraping of around 30sec for running of phpunit under drupal considered as 'normal' or is this a sign of something broken in my installation?
2. Is there any proposed way to reduce bootstraping time when doing only UnitTests?
3. I suppose I could create a second phpunit.xml, referring to an alternative core/tests/bootstrap.php with a $paths, which is reduced to our custom modules. And then, I would call phpunit with -c-option for the alternate phpunit.xml and the Unittest-Folder as argument. Is this a good idea or a sure way to shoot myself in the foot?

======
Update: Thanks to the Feedback from Kevin, I once more investigated why core/tests/bootstrap.php took so long to populate the classloader: The culprit is not bootstrap.php/drupal, but vagrant/virtualbox:

If I run the unit-tests from outside vagrant, they run in 1-2 seconds, as it seems normal to me (and is along what Kevin said)
If I run them inside the vagrant, the they take >30seconds.
The difference is in bootstrap.php, when drupal_phpunit_find_extension_directories() iterates over all Module-, Profile- and Theme-Directories in core, contrib and sites. Those directories are mounted via the Sync Folder-Feature by Vagrant, where there is a known performance-issue with Virtualbox.

Therefore, I renounce from hacking core :)

Answer 1

Kernel and Functional tests are expected to take longer because of a full Drupal bootstrap and site scaffolding per run to test in a clean environment. Unit tests are running raw code and should execute very quickly. I can run a few hundred of them in a matter of seconds.

Ensure your phpunit.xml file is set up correctly. For example, scanning for tests only in the custom module directory:

<testsuites>
    <testsuite name="unit">
      <directory>web/modules/custom/*/tests/src/Unit</directory>
    </testsuite>
    <testsuite name="kernel">
      <directory>web/modules/custom/*/tests/src/Kernel</directory>
    </testsuite>
    <testsuite name="functional">
      <directory>web/modules/custom/*/tests/src/Functional</directory>
    </testsuite>
  </testsuites>

Or running a specific kind of test:

phpunit --testsuite unit

Other than that something is wrong with your setup. It should never take 30 seconds to kickstart a unit test. Make sure you are not running with xdebug enabled and listening, etc.

(also, dont hack core!)