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

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

 /*
  * Copyright (C) 2011 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 1790 matching lines @@
 
     DCHECK(!flex_item.box->IsOutOfFlowPositioned());
 
     child->SetMayNeedPaintInvalidation();
 
     SetOverrideMainAxisContentSizeForChild(*child,
                                            flex_item.flexed_content_size);
     // 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);
     }
     // We may have already forced relayout for orthogonal flowing children in
     // computeInnerFlexBaseSizeForChild.
     bool force_child_relayout =
         relayout_children && !relaid_out_children_.Contains(child);
     if (child->IsLayoutBlock() &&
         ToLayoutBlock(*child).HasPercentHeightDescendants()) {
       // Have to force another relayout even though the child is sized
       // correctly, because its descendants are not sized correctly yet. Our
       // previous layout of the child was done without an override height set.
       // So, redo it here.
       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();
 
     UpdateAutoMarginsInMainAxis(*child, auto_margin_offset);
 
     LayoutUnit child_cross_axis_margin_box_extent;
     if (AlignmentForChild(*child) == kItemPositionBaseline &&
         !HasAutoMarginsInCrossAxis(*child)) {
       LayoutUnit ascent = MarginBoxAscentForChild(*child);
@@ skipping 356 matching lines @@
       LayoutUnit original_offset =
           line_contexts[line_number].cross_axis_offset - cross_axis_start_edge;
       LayoutUnit new_offset =
           content_extent - original_offset - line_cross_axis_extent;
       AdjustAlignmentForChild(*flex_item.box, new_offset - original_offset);
     }
   }
 }
 
 }  // namespace blink