diff --git a/pykolab/itip/__init__.py b/pykolab/itip/__init__.py
--- a/pykolab/itip/__init__.py
+++ b/pykolab/itip/__init__.py
@@ -172,9 +172,14 @@
         _iv = itip_event['xml']
         _ii = 0
 
-        while not conflict and _is is not None:
+        # T1988: For performance reasons we add "and _is < _ee" here
+        while not conflict and _is is not None and _is < _ee:
             # log.debug("* Comparing event dates at %s/%s with %s/%s" % (_es, _ee, _is, _ie), level=9)
             conflict = not _is_transparent(_ev) and not _is_transparent(_iv) and check_date_conflict(_es, _ee, _is, _ie)
+            # T1988: For performance reasons we modify recurrence iteration
+            if _is < _es:
+                _is = _is.replace(_es.year, _es.month, _es.day, 0, 0)
+
             _is = to_dt(itip_event['xml'].get_next_occurence(_is)) if itip_event['xml'].is_recurring() else None
             _ie = to_dt(itip_event['xml'].get_occurence_end_date(_is))
 
diff --git a/tests/unit/test-011-itip.py b/tests/unit/test-011-itip.py
--- a/tests/unit/test-011-itip.py
+++ b/tests/unit/test-011-itip.py
@@ -521,6 +521,22 @@
 
         self.assertFalse(itip.check_event_conflict(event, itip_event), "Conflicting dates (exception)")
 
+    def test_002_check_event_conflict_forever_recurring(self):
+        # This test is here to make sure performance issue is fixed (T1988)
+        # make the event recurring forever
+        itip_recurring_forever = itip_recurring.replace("RRULE:FREQ=DAILY;INTERVAL=1;COUNT=5", "RRULE:FREQ=WEEKLY;BYDAY=MO")
+        itip_event = itip.events_from_message(message_from_string(itip_recurring_forever))[0]
+
+        rrule = kolabformat.RecurrenceRule()
+        rrule.setFrequency(kolabformat.RecurrenceRule.Weekly)
+
+        event = Event()
+        event.set_recurrence(rrule)
+        event.set_start(datetime.datetime(2012, 6, 29, 9, 30, 0, tzinfo=pytz.utc))
+        event.set_end(datetime.datetime(2012, 6, 29, 10, 30, 0, tzinfo=pytz.utc))
+
+        self.assertFalse(itip.check_event_conflict(event, itip_event), "No conflict")
+
     def test_003_send_reply(self):
         itip_events = itip.events_from_message(message_from_string(itip_non_multipart))
         itip.send_reply("resource-collection-car@example.org", itip_events, "SUMMARY=%(summary)s; STATUS=%(status)s; NAME=%(name)s;")