Issue #3343913 by _shY, smustgrave, catch, xjm, quietone: Add comments...

Issue #3343913 by _shY, smustgrave, catch, xjm, quietone: Add comments explaining performance improvement in TypedData

(cherry picked from commit efecede1)

core/lib/Drupal/Core/TypedData/TypedData.php

@@ -175,6 +175,10 @@ public function getPropertyPath() {
       // The property path of this data object is the parent's path appended
       // by this object's name.
       $prefix = $this->parent->getPropertyPath();
+      // Variables in double quotes used to leverage fast string concatenation.
+      // In PHP 7+ concatenation with variable inside string is the fastest.
+      // @see https://blog.blackfire.io/php-7-performance-improvements-encapsed-strings-optimization.html
+      // This is being done because the code can run in the critical path.
       return $prefix !== '' ? "{$prefix}.{$this->name}" : $this->name;
     }
     // If no parent is set, this is the root of the data tree. Thus the property