diff --git a/openwisp_monitoring/check/classes/iperf.py b/openwisp_monitoring/check/classes/iperf.py
index b9797e905..5c10a45c4 100644
--- a/openwisp_monitoring/check/classes/iperf.py
+++ b/openwisp_monitoring/check/classes/iperf.py
@@ -26,6 +26,12 @@ def check(self, store=True):
             logger.warning(f'{device}: connection not properly set, Iperf skipped!')
             return
         device_connection.connect()
+        # We need to check device_connection is_working just right after connect().
+        # because it may be possible that authentication (publickey) failed.
+        # or any other failure happened during connect().
+        if not device_connection.is_working:
+            logger.warning(f'{device}: SSH connection is not working, Iperf skipped!')
+            return
         servers = self._get_iperf_servers(device.organization.id)
         command = f'iperf3 -c {servers[0]} -J'
         res, exit_code = device_connection.connector_instance.exec_command(
diff --git a/tests/openwisp2/settings.py b/tests/openwisp2/settings.py
index 636e9f7e1..d9d1c0941 100644
--- a/tests/openwisp2/settings.py
+++ b/tests/openwisp2/settings.py
@@ -170,7 +170,7 @@
 CELERY_BEAT_SCHEDULE = {
     'run_checks': {
         'task': 'openwisp_monitoring.check.tasks.run_checks',
-        'schedule': timedelta(minutes=5),
+        'schedule': timedelta(minutes=1),
         'args': None,
         'relative': True,
     }
@@ -274,7 +274,8 @@
     # Running on my local
     # Some Public Iperf Servers : https://iperf.fr/iperf-servers.php#public-servers
     # 'be63c4e5-a68a-4650-bfe8-733837edb8be': ['iperf.biznetnetworks.com'],
-    'a9734710-db30-46b0-a2fc-01f01046fe4f': ['speedtest.uztelecom.uz'],
+    # 'a9734710-db30-46b0-a2fc-01f01046fe4f': ['speedtest.uztelecom.uz'],
+    'a9734710-db30-46b0-a2fc-01f01046fe4f': ['192.168.5.109'],
     # '<org-pk>': ['<ORG_IPERF_SERVER>']
 }