Facter command slows down when phpunit is started over Jenkins - php

Question

I'm having the problem that shell_exec in combination with the facter command is very slow when started over Jenkins. Other commands than facter (like whoami) are fast.
The code runs on a Ubuntu VM that was recently upgrade from 14.x to 18.04.1 LTS. On Ubuntu 14.x I didn't encounter that problem. Facter is currently on version 3.11.3.
I nailed the speed problem down to the shell_exec in combination with facter by using the following code:
<?php
/**
*/
require_once 'AbstractTestCase.php';
/**
*/
class FacterTest extends AbstractTestCase
{
/**
*/
public function testSpeedDebug()
{
Core_Util_Debug::performanceStart('whoami');
shell_exec('whoami');
Core_Util_Debug::performanceEnd('whoami');
Core_Util_Debug::performanceStart('facter');
shell_exec('facter hostname');
Core_Util_Debug::performanceEnd('facter');
die (PHP_EOL);
}
}
When started manually over the CLI the output is:
>> name: whoami | time: 0.005261 s | memory: 3.3359 kB RAM
>> name: facter | time: 0.160292 s | memory: 0 B RAM
When started over Jenkins the output is:
>> name: whoami | time: 0.005495 s | memory: 3.3359 kB RAM
>> name: facter | time: 8.652776 s | memory: 0 B RAM
Does someone have an idea why it is slow when started over Jenkins (~8 times slower)?
Thank you in advance.
Extra info:
Gave it a quick try on Bamboo, and the behavior is the same as on Jenkins.