diff --git a/docs/Development.md b/docs/Development.md
index 19c6632..e2fce80 100644
--- a/docs/Development.md
+++ b/docs/Development.md
@@ -105,20 +105,22 @@ docker-compose exec php phing -f phing/tests.xml test:phpunit
 docker-compose run --rm phpunit --verbose --no-coverage
 ```
 
-# Mutation Tests
+# Infection - Mutation Testing
 
 Served by [Infection — Mutation Testing Framework](https://infection.github.io).
 
 ### Running tests
 
 ```bash
-docker-compose exec php vendor/bin/infection --threads=5
+$ docker-compose exec php bash
+root@18f2f0cfaa5d:/var/www/application# XDEBUG_MODE=coverage ./vendor/bin/infection --threads=$(nproc)
 ```
 
 or
 
 ```bash
-docker-compose exec php phing -f phing/tests.xml test:infection
+$ docker-compose exec php bash
+root@18f2f0cfaa5d:/var/www/application# XDEBUG_MODE=coverage phing -f phing/tests.xml test:infection
 ```
 
 ### Result of testing
diff --git a/phing/properties.dist b/phing/properties.dist
index c287955..76237a2 100644
--- a/phing/properties.dist
+++ b/phing/properties.dist
@@ -86,4 +86,4 @@ tests.database = ${dir.data.temporary}/database.sqlite
 # - Infection (mutation tests)
 #
 tests.phpunit.command = ./vendor/bin/phpunit --verbose
-tests.mutation.command = ./vendor/bin/infection --ansi --threads=5 --coverage=build/reports/infection
+tests.mutation.command = ./vendor/bin/infection --ansi --threads=$(nproc) --coverage=build/reports/infection