Alerting: Alert list performance improvements (#56247)

Co-authored-by: Gilles De Mey <gilles.de.mey@gmail.com>

.pa11yci-pr.conf.js

@@ -101,7 +101,7 @@ var config = {
       rootElement: '.main-view',
       // the unified alerting promotion alert's content contrast is too low
       // see https://github.com/grafana/grafana/pull/41829
-      threshold: 4,
+      threshold: 5,
     },
     {
       url: '${HOST}/datasources',