diff --git a/.env.default b/.env.default
index df58ae0..3f38c21 100644
--- a/.env.default
+++ b/.env.default
@@ -60,7 +60,8 @@ export WPT_SSH_PRIVATE_KEY_BASE64=""
 # Whether to enable debug Mode.
 #
 # Enabling debug mode will output verbose logging and details about each part
-# of the test runner.
+# of the test runner. It also runs npm install and build commands with
+# --loglevel=silly during preparation.
 #
 # 0 = Debug mode off
 # 1 = Debug mode on
diff --git a/README.md b/README.md
index 4302671..6e8b015 100644
--- a/README.md
+++ b/README.md
@@ -241,7 +241,7 @@ export WPT_SSH_OPTIONS=""
 export WPT_SSH_PRIVATE_KEY_BASE64=""
 
 # Output logging
-# Use 'verbose' to increase verbosity
+# Use 'verbose' to increase verbosity; also runs npm with --loglevel=silly
 export WPT_DEBUG=""
 
 # Certificate validation
@@ -395,7 +395,7 @@ export WPT_SSH_PRIVATE_KEY_BASE64=""
 
 **Output logging**
 
-Output logging. Use 'verbose' to increase verbosity.
+Output logging. Use 'verbose' to increase verbosity. When set, the npm install and build commands during preparation also run with `--loglevel=silly`.
 
 ```
 export WPT_DEBUG=""
diff --git a/prepare.php b/prepare.php
index 16d6d75..d503cfc 100644
--- a/prepare.php
+++ b/prepare.php
@@ -76,6 +76,13 @@
 }
 
 
+/*
+ * When debug mode is enabled, run npm at its most verbose logging level, so a
+ * hung or slow npm command shows what it is doing instead of sitting silent.
+ * The same debug mode adds verbosity to the rsync operations further down.
+ */
+$npm_verbosity = $runner_vars['WPT_DEBUG'] ? ' --loglevel=silly' : '';
+
 /*
  * Checkout and prepare wordpress-develop for testing.
  *
@@ -96,7 +103,7 @@
 		'git clone --depth=1 https://github.com/WordPress/wordpress-develop.git ' . escapeshellarg( $runner_vars['WPT_PREPARE_DIR'] ),
 
 		// Change directory to the preparation directory, install npm dependencies, and build the project.
-		'cd ' . escapeshellarg( $runner_vars['WPT_PREPARE_DIR'] ) . '; npm install && npm run build',
+		'cd ' . escapeshellarg( $runner_vars['WPT_PREPARE_DIR'] ) . '; npm install' . $npm_verbosity . ' && npm run build' . $npm_verbosity,
 
 	)
 );