--- a/layout/generic/nsFlexContainerFrame.cpp
+++ b/layout/generic/nsFlexContainerFrame.cpp
@@ -1739,16 +1739,19 @@ nsFlexContainerFrame::MeasureAscentAndBS
 
   aItem.Frame()->SetProperty(CachedFlexMeasuringReflow(), result);
   return *result;
 }
 
 /* virtual */ void
 nsFlexContainerFrame::MarkIntrinsicISizesDirty()
 {
+  mCachedMinISize = NS_INTRINSIC_WIDTH_UNKNOWN;
+  mCachedPrefISize = NS_INTRINSIC_WIDTH_UNKNOWN;
+
   for (nsIFrame* childFrame : mFrames) {
     childFrame->DeleteProperty(CachedFlexMeasuringReflow());
   }
   nsContainerFrame::MarkIntrinsicISizesDirty();
 }
 
 nscoord
 nsFlexContainerFrame::
@@ -5113,24 +5116,28 @@ nsFlexContainerFrame::IntrinsicISize(gfx
   }
 
   return containerISize;
 }
 
 /* virtual */ nscoord
 nsFlexContainerFrame::GetMinISize(gfxContext* aRenderingContext)
 {
-  nscoord minISize = 0;
-  DISPLAY_MIN_WIDTH(this, minISize);
-
-  minISize = IntrinsicISize(aRenderingContext, nsLayoutUtils::MIN_ISIZE);
-  return minISize;
+  DISPLAY_MIN_WIDTH(this, mCachedMinISize);
+  if (mCachedMinISize == NS_INTRINSIC_WIDTH_UNKNOWN) {
+    mCachedMinISize = IntrinsicISize(aRenderingContext,
+                                     nsLayoutUtils::MIN_ISIZE);
+  }
+
+  return mCachedMinISize;
 }
 
 /* virtual */ nscoord
 nsFlexContainerFrame::GetPrefISize(gfxContext* aRenderingContext)
 {
-  nscoord prefISize = 0;
-  DISPLAY_PREF_WIDTH(this, prefISize);
-
-  prefISize = IntrinsicISize(aRenderingContext, nsLayoutUtils::PREF_ISIZE);
-  return prefISize;
+  DISPLAY_PREF_WIDTH(this, mCachedPrefISize);
+  if (mCachedPrefISize == NS_INTRINSIC_WIDTH_UNKNOWN) {
+    mCachedPrefISize = IntrinsicISize(aRenderingContext,
+                                      nsLayoutUtils::PREF_ISIZE);
+  }
+
+  return mCachedPrefISize;
 }

--- a/layout/generic/nsFlexContainerFrame.h
+++ b/layout/generic/nsFlexContainerFrame.h
@@ -224,16 +224,18 @@ public:
    */
   static bool IsUsedFlexBasisContent(const nsStyleCoord* aFlexBasis,
                                      const nsStyleCoord* aMainSize);
 
 protected:
   // Protected constructor & destructor
   explicit nsFlexContainerFrame(ComputedStyle* aStyle)
     : nsContainerFrame(aStyle, kClassID)
+    , mCachedMinISize(NS_INTRINSIC_WIDTH_UNKNOWN)
+    , mCachedPrefISize(NS_INTRINSIC_WIDTH_UNKNOWN)
     , mBaselineFromLastReflow(NS_INTRINSIC_WIDTH_UNKNOWN)
     , mLastBaselineFromLastReflow(NS_INTRINSIC_WIDTH_UNKNOWN)
   {}
 
   virtual ~nsFlexContainerFrame();
 
   /*
    * This method does the bulk of the flex layout, implementing the algorithm
@@ -437,14 +439,20 @@ protected:
                           const nsSize& aContainerSize);
 
   /**
    * Helper for GetMinISize / GetPrefISize.
    */
   nscoord IntrinsicISize(gfxContext* aRenderingContext,
                          nsLayoutUtils::IntrinsicISizeType aType);
 
+  /**
+   * Cached values to optimize GetMinISize/GetPrefISize.
+   */
+  nscoord mCachedMinISize;
+  nscoord mCachedPrefISize;
+
   nscoord mBaselineFromLastReflow;
   // Note: the last baseline is a distance from our border-box end edge.
   nscoord mLastBaselineFromLastReflow;
 };
 
 #endif /* nsFlexContainerFrame_h___ */