diff --git a/dom/base/ChildIterator.cpp b/dom/base/ChildIterator.cpp
index 1929a05d1b3e..058e4b4aa450 100644
--- a/dom/base/ChildIterator.cpp
+++ b/dom/base/ChildIterator.cpp
@@ -124,6 +124,7 @@ void
 FlattenedChildIterator::Init(bool aIgnoreXBL)
 {
   if (aIgnoreXBL) {
+    mXBLInvolved = Some(false);
     return;
   }
 
@@ -132,7 +133,7 @@ FlattenedChildIterator::Init(bool aIgnoreXBL)
   if (mParent->IsElement()) {
     if (ShadowRoot* shadow = mParent->AsElement()->GetShadowRoot()) {
       mParent = shadow;
-      mXBLInvolved = true;
+      mXBLInvolved = Some(true);
       return;
     }
   }
@@ -143,25 +144,31 @@ FlattenedChildIterator::Init(bool aIgnoreXBL)
   if (binding) {
     MOZ_ASSERT(binding->GetAnonymousContent());
     mParent = binding->GetAnonymousContent();
-    mXBLInvolved = true;
+    mXBLInvolved = Some(true);
+  }
+}
+
+bool
+FlattenedChildIterator::ComputeWhetherXBLIsInvolved() const
+{
+  MOZ_ASSERT(mXBLInvolved.isNothing());
+  // We set mXBLInvolved to true if either the node we're iterating has a
+  // binding with content attached to it (in which case it is handled in Init),
+  // or the node is generated XBL content and has an <xbl:children> child.
+  if (!mParent->GetBindingParent()) {
+    return false;
   }
 
-  // We set mXBLInvolved to true if either:
-  // - The node we're iterating has a binding with content attached to it.
-  // - The node is generated XBL content and has an <xbl:children> child.
-  //
-  // FIXME(emilio): This is very slow :(
-  if (!mXBLInvolved && mParent->GetBindingParent()) {
-    for (nsIContent* child = mParent->GetFirstChild();
-         child;
-         child = child->GetNextSibling()) {
-      if (child->NodeInfo()->Equals(nsGkAtoms::children, kNameSpaceID_XBL)) {
-        MOZ_ASSERT(child->GetBindingParent());
-        mXBLInvolved = true;
-        break;
-      }
+  for (nsIContent* child = mParent->GetFirstChild();
+       child;
+       child = child->GetNextSibling()) {
+    if (child->NodeInfo()->Equals(nsGkAtoms::children, kNameSpaceID_XBL)) {
+      MOZ_ASSERT(child->GetBindingParent());
+      return true;
     }
   }
+
+  return false;
 }
 
 bool
diff --git a/dom/base/ChildIterator.h b/dom/base/ChildIterator.h
index 2b20b4e12e71..be95b8a54eee 100644
--- a/dom/base/ChildIterator.h
+++ b/dom/base/ChildIterator.h
@@ -134,7 +134,6 @@ public:
   explicit FlattenedChildIterator(const nsIContent* aParent,
                                   bool aStartAtBeginning = true)
     : ExplicitChildIterator(aParent, aStartAtBeginning)
-    , mXBLInvolved(false)
     , mOriginalContent(aParent)
   {
     Init(false);
@@ -142,20 +141,30 @@ public:
 
   FlattenedChildIterator(FlattenedChildIterator&& aOther)
     : ExplicitChildIterator(Move(aOther))
-    , mXBLInvolved(aOther.mXBLInvolved)
     , mOriginalContent(aOther.mOriginalContent)
+    , mXBLInvolved(aOther.mXBLInvolved)
   {}
 
   FlattenedChildIterator(const FlattenedChildIterator& aOther)
     : ExplicitChildIterator(aOther)
-    , mXBLInvolved(aOther.mXBLInvolved)
     , mOriginalContent(aOther.mOriginalContent)
+    , mXBLInvolved(aOther.mXBLInvolved)
   {}
 
-  bool XBLInvolved() { return mXBLInvolved; }
+  bool XBLInvolved() {
+    if (mXBLInvolved.isNothing()) {
+      mXBLInvolved = Some(ComputeWhetherXBLIsInvolved());
+    }
+    return *mXBLInvolved;
+  }
 
   const nsIContent* Parent() const { return mOriginalContent; }
 
+private:
+  bool ComputeWhetherXBLIsInvolved() const;
+
+  void Init(bool aIgnoreXBL);
+
 protected:
   /**
    * This constructor is a hack to help AllChildrenIterator which sometimes
@@ -164,20 +173,20 @@ protected:
   FlattenedChildIterator(const nsIContent* aParent, uint32_t aFlags,
                          bool aStartAtBeginning = true)
     : ExplicitChildIterator(aParent, aStartAtBeginning)
-    , mXBLInvolved(false)
     , mOriginalContent(aParent)
   {
     bool ignoreXBL = aFlags & nsIContent::eAllButXBL;
     Init(ignoreXBL);
   }
 
-  void Init(bool aIgnoreXBL);
-
-  // For certain optimizations, nsCSSFrameConstructor needs to know if the
-  // child list of the element that we're iterating matches its .childNodes.
-  bool mXBLInvolved;
-
   const nsIContent* mOriginalContent;
+
+private:
+  // For certain optimizations, nsCSSFrameConstructor needs to know if the child
+  // list of the element that we're iterating matches its .childNodes.
+  //
+  // This is lazily computed when asked for it.
+  Maybe<bool> mXBLInvolved;
 };
 
 /**