Search - enable low_level_cancellation by default. (#42291) (#42857)

Benchmarking on worst-case queries (max agg on match_all or popular-term query with large index) was not noticeably slower.

Closes #26258

server/src/main/java/org/elasticsearch/search/SearchService.java

@@ -134,11 +134,11 @@ public class SearchService extends AbstractLifecycleComponent implements IndexEv
 
     /**
      * Enables low-level, frequent search cancellation checks. Enabling low-level checks will make long running searches to react
-     * to the cancellation request faster. However, since it will produce more cancellation checks it might slow the search performance
-     * down.
+     * to the cancellation request faster. It will produce more cancellation checks but benchmarking has shown these did not 
+     * noticeably slow down searches.
      */
     public static final Setting<Boolean> LOW_LEVEL_CANCELLATION_SETTING =
-        Setting.boolSetting("search.low_level_cancellation", false, Property.Dynamic, Property.NodeScope);
+        Setting.boolSetting("search.low_level_cancellation", true, Property.Dynamic, Property.NodeScope);
 
     public static final TimeValue NO_TIMEOUT = timeValueMillis(-1);
     public static final Setting<TimeValue> DEFAULT_SEARCH_TIMEOUT_SETTING =