*** DO NOT LAND *** Avoid multiple layout passes in flexbox.

third_party/WebKit/Source/core/layout/LayoutFlexibleBox.cpp

Index: third_party/WebKit/Source/core/layout/LayoutFlexibleBox.cpp
diff --git a/third_party/WebKit/Source/core/layout/LayoutFlexibleBox.cpp b/third_party/WebKit/Source/core/layout/LayoutFlexibleBox.cpp
index 8aff1ea9f21be17d039296fd1b58b7aea2b8e0e3..c7b21ade18be8d55ca7ff85ddc248c8755075e54 100644
--- a/third_party/WebKit/Source/core/layout/LayoutFlexibleBox.cpp
+++ b/third_party/WebKit/Source/core/layout/LayoutFlexibleBox.cpp
@@ -1808,10 +1808,13 @@ void LayoutFlexibleBox::LayoutAndPlaceChildren(
     // The flexed content size and the override size include the scrollbar
     // width, so we need to compare to the size including the scrollbar.
     // TODO(cbiesinger): Should it include the scrollbar?
+#if 0  // Disabling this eliminates most of the slowness.
     if (flex_item.flexed_content_size !=
         MainAxisContentExtentForChildIncludingScrollbar(*child)) {
       child->SetChildNeedsLayout(kMarkOnlyThis);
-    } else {
+    } else
+#endif
+    {
       // To avoid double applying margin changes in
       // updateAutoMarginsInCrossAxis, we reset the margins here.
       ResetAutoMarginsAndLogicalTopInCrossAxis(*child);
@@ -1829,8 +1832,10 @@ void LayoutFlexibleBox::LayoutAndPlaceChildren(
       force_child_relayout = true;
     }
     UpdateBlockChildDirtyBitsBeforeLayout(force_child_relayout, *child);
+#if 0  // Disabling this also improves speed somewhat.
     if (!child->NeedsLayout())
       MarkChildForPaginationRelayoutIfNeeded(*child, layout_scope);
+#endif
     if (child->NeedsLayout())
       relaid_out_children_.insert(child);
     child->LayoutIfNeeded();