diff --git a/js/src/tests/jstests.py b/js/src/tests/jstests.py
index a00c88a6915c..7f186896323d 100755
--- a/js/src/tests/jstests.py
+++ b/js/src/tests/jstests.py
@@ -66,6 +66,12 @@ def parse_args():
     harness_og.add_option('-t', '--timeout', type=float, default=150.0,
                           help='Set maximum time a test is allows to run'
                           ' (in seconds).')
+    harness_og.add_option('--show-slow', action='store_true',
+                          help='Show tests taking longer than a minimum time'
+                          ' (in seconds).')
+    harness_og.add_option('--slow-test-threshold', type=float, default=5.0,
+                          help='Time in seconds a test can take until it is'
+                          'considered slow (default %default).')
     harness_og.add_option('-a', '--args', dest='shell_args', default='',
                           help='Extra args to pass to the JS shell.')
     harness_og.add_option('--jitflags', dest='jitflags', default='none',
diff --git a/js/src/tests/lib/results.py b/js/src/tests/lib/results.py
index 451fe5b657be..3265cf4dcadf 100644
--- a/js/src/tests/lib/results.py
+++ b/js/src/tests/lib/results.py
@@ -94,6 +94,11 @@ class TestResult:
 
         return cls(test, result, results)
 
+class TestDuration:
+    def __init__(self, test, duration):
+        self.test = test
+        self.duration = duration
+
 class ResultsSink:
     def __init__(self, options, testcount):
         self.options = options
@@ -102,6 +107,7 @@ class ResultsSink:
         self.groups = {}
         self.output_dict = {}
         self.counts = {'PASS': 0, 'FAIL': 0, 'TIMEOUT': 0, 'SKIP': 0}
+        self.slow_tests = []
         self.n = 0
 
         if options.hide_progress:
@@ -116,6 +122,8 @@ class ResultsSink:
             self.pb = ProgressBar(testcount, fmt)
 
     def push(self, output):
+        if self.options.show_slow and output.dt >= self.options.slow_test_threshold:
+            self.slow_tests.append(TestDuration(output.test, output.dt))
         if output.timed_out:
             self.counts['TIMEOUT'] += 1
         if isinstance(output, NullTestOutput):
@@ -257,6 +265,17 @@ class ResultsSink:
         else:
             print('FAIL' + suffix)
 
+        if self.options.show_slow:
+            min_duration = self.options.slow_test_threshold
+            print('Slow tests (duration > {}s)'.format(min_duration))
+            slow_tests = sorted(self.slow_tests, key=lambda x: x.duration, reverse=True)
+            any = False
+            for test in slow_tests:
+                print('{:>5} {}'.format(round(test.duration, 2), test.test))
+                any = True
+            if not any:
+                print('None')
+
     def all_passed(self):
         return 'REGRESSIONS' not in self.groups and 'TIMEOUTS' not in self.groups