diff --git a/src/mscorlib/src/System/Globalization/IdnMapping.cs b/src/mscorlib/src/System/Globalization/IdnMapping.cs
index cbc0e7988835..7881ac47410f 100644
--- a/src/mscorlib/src/System/Globalization/IdnMapping.cs
+++ b/src/mscorlib/src/System/Globalization/IdnMapping.cs
@@ -710,8 +710,10 @@ static String punycode_encode(String unicode)
                                             k >= bias + tmax ? tmax : k - bias;
                                     if (q < t) break;
                                     Contract.Assert(punycodeBase != t, "[IdnMapping.punycode_encode]Expected punycodeBase (36) to be != t");
-                                    output.Append(encode_digit(t + (q - t) % (punycodeBase - t)));
-                                    q = (q - t) / (punycodeBase - t);
+
+                                    int mod;
+                                    q = Math.DivRem(q - t, punycodeBase - t, out mod);
+                                    output.Append(encode_digit(t + mod));
                                 }
 
                                 output.Append(encode_digit(q));
diff --git a/src/mscorlib/src/System/Math.cs b/src/mscorlib/src/System/Math.cs
index cf2b0ee15c31..9ff0a9f6c898 100644
--- a/src/mscorlib/src/System/Math.cs
+++ b/src/mscorlib/src/System/Math.cs
@@ -726,13 +726,21 @@ public static long BigMul(int a, int b) {
         }
 
         public static int DivRem(int a, int b, out int result) {
-            result =  a%b;
-            return a/b;
+            // TODO https://github.com/dotnet/coreclr/issues/3439:
+            // Restore to using % and / when the JIT is able to eliminate one of the idivs.
+            // In the meantime, a * and - is measurably faster than an extra /.
+            int div = a / b;
+            result = a - (div * b);
+            return div;
         }
 
         public static long DivRem(long a, long b, out long result) {
-            result =  a%b;
-            return a/b;
+            // TODO https://github.com/dotnet/coreclr/issues/3439:
+            // Restore to using % and / when the JIT is able to eliminate one of the idivs.
+            // In the meantime, a * and - is measurably faster than an extra /.
+            long div = a / b;
+            result = a - (div * b);
+            return div;
         }
     }
 }