diff --git a/calendar/classes/local/event/strategies/raw_event_retrieval_strategy.php b/calendar/classes/local/event/strategies/raw_event_retrieval_strategy.php
index 37d1a8038e2..f442a64a15c 100644
--- a/calendar/classes/local/event/strategies/raw_event_retrieval_strategy.php
+++ b/calendar/classes/local/event/strategies/raw_event_retrieval_strategy.php
@@ -278,7 +278,12 @@ class raw_event_retrieval_strategy implements raw_event_retrieval_strategy_inter
 
         // Build the WHERE condition for the sub-query.
         if (!empty($subqueryconditions)) {
-            $subquerywhere = 'WHERE ' . implode(" OR ", $subqueryconditions);
+            $unionstartquery = "SELECT modulename, instance, eventtype, priority
+                                  FROM {event} ev
+                                 WHERE ";
+            $subqueryunion = $unionstartquery . implode(" UNION $unionstartquery ", $subqueryconditions);
+        } else {
+            $subqueryunion = '{event}';
         }
 
         // Merge subquery parameters to the parameters of the main query.
@@ -291,8 +296,7 @@ class raw_event_retrieval_strategy implements raw_event_retrieval_strategy_inter
                             ev.instance,
                             ev.eventtype,
                             MIN(ev.priority) as priority
-                       FROM {event} ev
-                      $subquerywhere
+                       FROM ($subqueryunion) ev
                    GROUP BY ev.modulename, ev.instance, ev.eventtype";
 
         // Build the main query.
diff --git a/lib/db/install.xml b/lib/db/install.xml
index fcef7da2b25..9a30e74c4f5 100644
--- a/lib/db/install.xml
+++ b/lib/db/install.xml
@@ -461,6 +461,8 @@
         <INDEX NAME="uuid" UNIQUE="false" FIELDS="uuid"/>
         <INDEX NAME="type-timesort" UNIQUE="false" FIELDS="type, timesort"/>
         <INDEX NAME="groupid-courseid-categoryid-visible-userid" UNIQUE="false" FIELDS="groupid, courseid, categoryid, visible, userid" COMMENT="used for calendar view"/>
+        <INDEX NAME="eventtype" UNIQUE="false" FIELDS="eventtype"/>
+        <INDEX NAME="modulename-instance" UNIQUE="false" FIELDS="modulename, instance"/>
       </INDEXES>
     </TABLE>
     <TABLE NAME="cache_filters" COMMENT="For keeping information about cached data">
diff --git a/lib/db/upgrade.php b/lib/db/upgrade.php
index 4c56b3969c5..a80612c7341 100644
--- a/lib/db/upgrade.php
+++ b/lib/db/upgrade.php
@@ -2145,5 +2145,22 @@ function xmldb_main_upgrade($oldversion) {
         upgrade_main_savepoint(true, 2019122000.01);
     }
 
+    if ($oldversion < 2020010900.02) {
+        $table = new xmldb_table('event');
+
+        // This index will improve the performance when the Events API retrieves category and group events.
+        $index = new xmldb_index('eventtype', XMLDB_INDEX_NOTUNIQUE, ['eventtype']);
+        if (!$dbman->index_exists($table, $index)) {
+            $dbman->add_index($table, $index);
+        }
+
+        // This index improves the performance of backups, deletion and visibilty changes on activities.
+        $index = new xmldb_index('modulename-instance', XMLDB_INDEX_NOTUNIQUE, ['modulename', 'instance']);
+        if (!$dbman->index_exists($table, $index)) {
+            $dbman->add_index($table, $index);
+        }
+
+        upgrade_main_savepoint(true, 2020010900.02);
+    }
     return true;
 }