Work around performance issue in vobject library

src/pretix/helpers/apps.py

@@ -25,3 +25,7 @@ from django.apps import AppConfig
 class PretixHelpersConfig(AppConfig):
     name = 'pretix.helpers'
     label = 'pretixhelpers'
+
+    def ready(self):
+        from .monkeypatching import monkeypatch_all_at_ready
+        monkeypatch_all_at_ready()

src/pretix/helpers/monkeypatching.py

@@ -0,0 +1,35 @@
+from datetime import datetime
+
+
+def monkeypatch_vobject_performance():
+    """
+    This works around a performance issue in the unmaintained vobject library which calls
+    a very expensive function for every event in a calendar. Since the slow function is
+    mostly used to compare timezones to UTC, not to arbitrary other timezones, we can
+    add a few early-out optimizations.
+    """
+
+    from vobject import icalendar
+
+    old_tzinfo_eq = icalendar.tzinfo_eq
+    test_date = datetime(2000, 1, 1)
+
+    def new_tzinfo_eq(tzinfo1, tzinfo2, *args, **kwargs):
+        if tzinfo1 is None:
+            return tzinfo2 is None
+        if tzinfo2 is None:
+            return tzinfo1 is None
+
+        n1 = tzinfo1.tzname(test_date)
+        n2 = tzinfo2.tzname(test_date)
+        if n1 == "UTC" and n2 == "UTC":
+            return True
+        if n1 == "UTC" or n2 == "UTC":
+            return False
+        return old_tzinfo_eq(tzinfo1, tzinfo2, *args, **kwargs)
+
+    icalendar.tzinfo_eq = new_tzinfo_eq
+
+
+def monkeypatch_all_at_ready():
+    monkeypatch_vobject_performance()