diff --git a/gcal_sync/sync.py b/gcal_sync/sync.py
index e9aafbe..cb7a7ae 100644
--- a/gcal_sync/sync.py
+++ b/gcal_sync/sync.py
@@ -32,12 +32,12 @@ class CalendarSync():
 
         def get_key(item): return item[key]
 
-        keys_src = list(map(get_key, items_src))
-        keys_dst = list(map(get_key, items_dst))
+        keys_src = set(map(get_key, items_src))
+        keys_dst = set(map(get_key, items_dst))
 
-        keys_to_insert = set(keys_src) - set(keys_dst)
-        keys_to_update = set(keys_src) & set(keys_dst)
-        keys_to_delete = set(keys_dst) - set(keys_src)
+        keys_to_insert = keys_src - keys_dst
+        keys_to_update = keys_src & keys_dst
+        keys_to_delete = keys_dst - keys_src
 
         def get_item(items, key_val):
             for item in filter(lambda item: item[key] == key_val, items):