diff --git a/src/libraries/System.Private.CoreLib/src/System/Span.cs b/src/libraries/System.Private.CoreLib/src/System/Span.cs
index 1c66a341b0fde1..d8fbb261e4f381 100644
--- a/src/libraries/System.Private.CoreLib/src/System/Span.cs
+++ b/src/libraries/System.Private.CoreLib/src/System/Span.cs
@@ -300,6 +300,15 @@ public unsafe void Clear()
         [MethodImpl(MethodImplOptions.AggressiveInlining)]
         public unsafe void Fill(T value)
         {
+            // Mono works faster with Unsafe.InitBlockUnaligned
+#if MONO
+            if (sizeof(T) == 1)
+            {
+                Unsafe.InitBlockUnaligned(ref Unsafe.As<T, byte>(ref _reference), *(byte*)&value, (uint)_length);
+                return;
+            }
+#endif
+
             SpanHelpers.Fill(ref _reference, (uint)_length, value);
         }
 
diff --git a/src/libraries/System.Private.CoreLib/src/System/SpanHelpers.ByteMemOps.cs b/src/libraries/System.Private.CoreLib/src/System/SpanHelpers.ByteMemOps.cs
index 54294592099256..2a47b9437ecec2 100644
--- a/src/libraries/System.Private.CoreLib/src/System/SpanHelpers.ByteMemOps.cs
+++ b/src/libraries/System.Private.CoreLib/src/System/SpanHelpers.ByteMemOps.cs
@@ -254,6 +254,13 @@ public static unsafe void ClearWithoutReferences(ref byte dest, nuint len)
             if (len == 0)
                 return;
 
+            // Mono works faster with Unsafe.InitBlockUnaligned
+#if MONO
+            if (len > ZeroMemoryNativeThreshold)
+                goto PInvoke;
+            Unsafe.InitBlockUnaligned(ref dest, 0, (uint)len);
+            return;
+#else
             ref byte destEnd = ref Unsafe.Add(ref dest, len);
 
             if (len <= 16)
@@ -427,6 +434,7 @@ public static unsafe void ClearWithoutReferences(ref byte dest, nuint len)
             Unsafe.WriteUnaligned<int>(ref Unsafe.Add(ref destEnd, -4), 0);
 #endif
             return;
+#endif // MONO
 
         PInvoke:
             // Implicit nullchecks
@@ -498,24 +506,16 @@ internal static void Fill(ref byte dest, byte value, nuint len)
                 nuint stopLoopAtOffset = len & ~(nuint)7;
                 do
                 {
-                    Unsafe.Add(ref dest, (nint)i + 0) = value;
-                    Unsafe.Add(ref dest, (nint)i + 1) = value;
-                    Unsafe.Add(ref dest, (nint)i + 2) = value;
-                    Unsafe.Add(ref dest, (nint)i + 3) = value;
-                    Unsafe.Add(ref dest, (nint)i + 4) = value;
-                    Unsafe.Add(ref dest, (nint)i + 5) = value;
-                    Unsafe.Add(ref dest, (nint)i + 6) = value;
-                    Unsafe.Add(ref dest, (nint)i + 7) = value;
+                    // broadcast the value to all 8 bytes of the ulong and write it to memory
+                    Unsafe.WriteUnaligned(ref Unsafe.Add(ref dest, i), value * 0x101010101010101ul);
                 } while ((i += 8) < stopLoopAtOffset);
             }
 
             // Write next 4 elements if needed
             if ((len & 4) != 0)
             {
-                Unsafe.Add(ref dest, (nint)i + 0) = value;
-                Unsafe.Add(ref dest, (nint)i + 1) = value;
-                Unsafe.Add(ref dest, (nint)i + 2) = value;
-                Unsafe.Add(ref dest, (nint)i + 3) = value;
+                // broadcast the value to all 4 bytes of the uint and write it to memory
+                Unsafe.WriteUnaligned(ref Unsafe.Add(ref dest, i), value * 0x1010101u);
                 i += 4;
             }