diff --git a/src/inventory/BaseInventory.php b/src/inventory/BaseInventory.php
index 433fcbf87..308850ffd 100644
--- a/src/inventory/BaseInventory.php
+++ b/src/inventory/BaseInventory.php
@@ -172,6 +172,10 @@ abstract class BaseInventory implements Inventory{
 		return -1;
 	}
 
+	/**
+	 * TODO: make this abstract and force implementations to implement it properly (BC break)
+	 * This default implementation works, but is slow.
+	 */
 	public function isSlotEmpty(int $index) : bool{
 		return $this->getItem($index)->isNull();
 	}