Added a description how to speed up testing with parallel execution

content/doc/developer/testing/index.adoc

@@ -148,6 +148,36 @@ Most Java IDEs should be able to run JUnit tests and report on the results.
 //==== From the Command Line
 //==== From an IDE
 
+=== Performance considerations
+
+The test runner (Surefire) supports running tests in parallel to speed them up.
+A possibility to configure this machine-wide is to adjust the `forkCount` within Maven.
+The default setting is `forkCount=1`, which means no parallel testing.
+An often-used setting is `1C`, which spawns as many testing processes in parallel as CPU cores are present.
+If you have a machine with many cores, it might be faster to set it to a smaller number, like `0.45C`.
+For example, with 16 cores the runner will be spawning up to 7 processes.
+More details can be found in the link:https://maven.apache.org/surefire/maven-surefire-plugin/examples/fork-options-and-parallel-execution.html[Maven Surefire documentation].
+
+You can also adjust this globally within a Maven profile setting, independently of the Plugin configuration.
+The Maven profile can be typically found at: `~/.m2/settings.xml` (Linux) or `%userprofile%/.m2` (Windows).
+A profile setting with the name "faster", which is enabled by default can look like this:
+
+[source,xml]
+----
+<profile>
+  <id>faster</id>
+  <activation>
+    <activeByDefault>true</activeByDefault>
+  </activation>
+  <properties>
+    <forkCount>0.45C</forkCount>
+  </properties>
+</profile>
+----
+
+In the IDE this setting will be able to enable or disable depending on the test suite you are working on.
+In case of problems it can also be deactivated on the command-line with `-P=-faster`.
+
 == What to Test
 Now that we can write a basic test, we should discuss what you should be testing…