diff --git a/dom/base/CustomElementRegistry.cpp b/dom/base/CustomElementRegistry.cpp
index 11ba39a67e..bb10f3ca80 100644
--- a/dom/base/CustomElementRegistry.cpp
+++ b/dom/base/CustomElementRegistry.cpp
@@ -1278,6 +1278,9 @@ CustomElementDefinition::CustomElementDefinition(nsIAtom* aType,
     mCallbacks(aCallbacks),
     mDocOrder(aDocOrder)
 {
+  for (uint32_t i = 0; i < mObservedAttributes.Length(); ++i) {
+    mObservedAttributesSet.PutEntry(mObservedAttributes[i]);
+  }
 }
 
 //-----------------------------------------------------
diff --git a/dom/base/CustomElementRegistry.h b/dom/base/CustomElementRegistry.h
index 06e5947dbc..53f58b5a9f 100644
--- a/dom/base/CustomElementRegistry.h
+++ b/dom/base/CustomElementRegistry.h
@@ -16,6 +16,8 @@
 #include "mozilla/dom/WebComponentsBinding.h"
 #include "nsCycleCollectionParticipant.h"
 #include "nsGenericHTMLElement.h"
+#include "nsHashKeys.h"
+#include "nsTHashtable.h"
 #include "nsWrapperCache.h"
 
 class nsDocument;
@@ -166,6 +168,9 @@ struct CustomElementDefinition
   // The list of attributes that this custom element observes.
   nsCOMArray<nsIAtom> mObservedAttributes;
 
+  // Hash set for O(1) lookup of observed attributes.
+  nsTHashtable<nsISupportsHashKey> mObservedAttributesSet;
+
   // The prototype to use for new custom elements of this type.
   JS::Heap<JSObject *> mPrototype;
 
@@ -189,7 +194,7 @@ struct CustomElementDefinition
       return false;
     }
 
-    return mObservedAttributes.Contains(aName);
+    return mObservedAttributesSet.Contains(aName);
   }
 
 private:
diff --git a/dom/base/ShadowRoot.cpp b/dom/base/ShadowRoot.cpp
index f2ae953a0e..d2da59edbb 100644
--- a/dom/base/ShadowRoot.cpp
+++ b/dom/base/ShadowRoot.cpp
@@ -136,7 +136,7 @@ ShadowRoot::AddSlot(HTMLSlotElement* aSlot)
   MOZ_ASSERT(aSlot);
 
   // Note that if name attribute missing, the slot is a default slot.
- nsAutoString name;
+  nsAutoString name;
   aSlot->GetName(name);
 
   nsTArray<HTMLSlotElement*>* currentSlots = mSlotMap.LookupOrAdd(name);
@@ -162,25 +162,14 @@ ShadowRoot::AddSlot(HTMLSlotElement* aSlot)
 
       oldSlot->RemoveAssignedNode(assignedNode);
       currentSlot->AppendAssignedNode(assignedNode);
-
-      Element* restyleElement;
-      if (assignedNode->IsElement()) {
-        restyleElement = assignedNode->AsElement();
-      } else {
-        // This is likely a text node. Use the host instead.
-        restyleElement = GetHost();
-      }
-      if (restyleElement) {
-        nsLayoutUtils::PostRestyleEvent(
-          restyleElement, eRestyle_Subtree, nsChangeHint(0));
-      }
-
       doEnqueueSlotChange = true;
     }
 
     if (doEnqueueSlotChange) {
       oldSlot->EnqueueSlotChangeEvent();
       currentSlot->EnqueueSlotChangeEvent();
+      nsLayoutUtils::PostRestyleEvent(
+        GetHost(), eRestyle_Subtree, nsChangeHint(0));
     }
   } else {
     // Otherwise add appropriate nodes to this slot from the host.
@@ -475,7 +464,10 @@ ShadowRoot::MaybeReassignElement(Element* aElement,
 void
 ShadowRoot::DistributionChanged()
 {
-  // FIXME(emilio): We could be more granular in a bunch of cases.
+  if (mSlotMap.IsEmpty()) {
+    return;
+  }
+
   auto* host = GetHost();
   if (!host || !host->IsInComposedDoc()) {
     return;
@@ -551,8 +543,11 @@ ShadowRoot::AttributeChanged(nsIDocument* aDocument,
     return;
   }
 
-  //XXX optimize this!
-  shell->DestroyFramesForAndRestyle(aElement);
+  if (aElement->GetPrimaryFrame()) {
+    shell->DestroyFramesForAndRestyle(aElement);
+  } else {
+    nsLayoutUtils::PostRestyleEvent(aElement, eRestyle_Subtree, nsChangeHint(0));
+  }
 }
 
 void
@@ -564,7 +559,7 @@ ShadowRoot::ContentAppended(nsIDocument* aDocument,
   for (nsIContent* content = aFirstNewContent;
        content;
        content = content->GetNextSibling()) {
-    ContentInserted(aDocument, aContainer, aFirstNewContent, aNewIndexInContainer);
+    ContentInserted(aDocument, aContainer, content, aNewIndexInContainer);
   }
 }