I didn't like this change, but missed it in the review. Readability over performance in this case. Not to mention that the JVM is likely to hoist the instanceof check and cast out of the loop because it is final invariant.

src/main/java/com/zaxxer/hikari/util/PropertyElf.java

@@ -49,11 +49,6 @@ public final class PropertyElf
          return;
       }
 
-      HikariConfig config = null; 
-      if (target instanceof HikariConfig) {
-         config = (HikariConfig) target;
-      }
-
       List<Method> methods = Arrays.asList(target.getClass().getMethods());
       Enumeration<?> propertyNames = properties.propertyNames();
       while (propertyNames.hasMoreElements()) {
@@ -64,8 +59,8 @@ public final class PropertyElf
             propValue = properties.get(key);
          }
 
-         if (config != null && propName.startsWith("dataSource.")) {
+         if (target instanceof HikariConfig && propName.startsWith("dataSource.")) {
-            config.addDataSourceProperty(propName.substring("dataSource.".length()), propValue);
+            ((HikariConfig) target).addDataSourceProperty(propName.substring("dataSource.".length()), propValue);
          }
          else {
             setProperty(target, propName, propValue, methods);