diff --git a/src/libraries/System.Private.Uri/src/System/IriHelper.cs b/src/libraries/System.Private.Uri/src/System/IriHelper.cs
index bb18ff9485c94..5b821405717b0 100644
--- a/src/libraries/System.Private.Uri/src/System/IriHelper.cs
+++ b/src/libraries/System.Private.Uri/src/System/IriHelper.cs
@@ -109,11 +109,19 @@ internal static unsafe string EscapeUnescapeIri(char* pInput, int start, int end
             ValueStringBuilder dest = new ValueStringBuilder(size);
             byte[]? bytes = null;
 
+            const int percentEncodingLen = 3; // Escaped UTF-8 will take 3 chars: %AB.
+            int bufferRemaining = 0;
+
             int next = start;
             char ch;
+            bool escape = false;
+            bool surrogatePair = false;
 
             for (; next < end; ++next)
             {
+                escape = false;
+                surrogatePair = false;
+
                 if ((ch = pInput[next]) == '%')
                 {
                     if (next + 2 < end)
@@ -218,55 +226,31 @@ internal static unsafe string EscapeUnescapeIri(char* pInput, int start, int end
                 {
                     // unicode
 
-                    bool escape;
-                    bool surrogatePair = false;
-
-                    char ch2 = '\0';
+                    char ch2;
 
                     if ((char.IsHighSurrogate(ch)) && (next + 1 < end))
                     {
                         ch2 = pInput[next + 1];
                         escape = !CheckIriUnicodeRange(ch, ch2, ref surrogatePair, component == UriComponents.Query);
-                    }
-                    else
-                    {
-                        escape = !CheckIriUnicodeRange(ch, component == UriComponents.Query);
-                    }
-
-                    if (escape)
-                    {
-                        Span<byte> encodedBytes = stackalloc byte[4];
-
-                        Rune rune;
-                        if (surrogatePair)
-                        {
-                            rune = new Rune(ch, ch2);
-                        }
-                        else if (!Rune.TryCreate(ch, out rune))
+                        if (!escape)
                         {
-                            rune = Rune.ReplacementChar;
-                        }
-
-                        int bytesWritten = rune.EncodeToUtf8(encodedBytes);
-                        encodedBytes = encodedBytes.Slice(0, bytesWritten);
-
-                        foreach (byte b in encodedBytes)
-                        {
-                            UriHelper.EscapeAsciiChar(b, ref dest);
+                            // copy the two chars
+                            dest.Append(pInput[next++]);
+                            dest.Append(pInput[next]);
                         }
                     }
                     else
                     {
-                        dest.Append(ch);
-                        if (surrogatePair)
+                        if (CheckIriUnicodeRange(ch, component == UriComponents.Query))
                         {
-                            dest.Append(ch2);
+                            // copy it
+                            dest.Append(pInput[next]);
+                        }
+                        else
+                        {
+                            // escape it
+                            escape = true;
                         }
-                    }
-
-                    if (surrogatePair)
-                    {
-                        next++;
                     }
                 }
                 else
@@ -274,6 +258,25 @@ internal static unsafe string EscapeUnescapeIri(char* pInput, int start, int end
                     // just copy the character
                     dest.Append(pInput[next]);
                 }
+
+                if (escape)
+                {
+                    const int MaxNumberOfBytesEncoded = 4;
+
+                    byte[] encodedBytes = new byte[MaxNumberOfBytesEncoded];
+                    fixed (byte* pEncodedBytes = &encodedBytes[0])
+                    {
+                        int encodedBytesCount = Encoding.UTF8.GetBytes(pInput + next, surrogatePair ? 2 : 1, pEncodedBytes, MaxNumberOfBytesEncoded);
+                        Debug.Assert(encodedBytesCount <= MaxNumberOfBytesEncoded, "UTF8 encoder should not exceed specified byteCount");
+
+                        bufferRemaining -= encodedBytesCount * percentEncodingLen;
+
+                        for (int count = 0; count < encodedBytesCount; ++count)
+                        {
+                            UriHelper.EscapeAsciiChar(encodedBytes[count], ref dest);
+                        }
+                    }
+                }
             }
 
             string result = dest.ToString();
diff --git a/src/libraries/System.Private.Uri/tests/FunctionalTests/EscapeUnescapeIriTests.cs b/src/libraries/System.Private.Uri/tests/FunctionalTests/EscapeUnescapeIriTests.cs
deleted file mode 100644
index db095dd3da78d..0000000000000
--- a/src/libraries/System.Private.Uri/tests/FunctionalTests/EscapeUnescapeIriTests.cs
+++ /dev/null
@@ -1,33 +0,0 @@
-using System.Collections.Generic;
-using Xunit;
-
-namespace System.PrivateUri.Tests
-{
-    public class EscapeUnescapeIriTests
-    {
-        public static IEnumerable<object[]> ReplacesStandaloneSurrogatesWithReplacementChar()
-        {
-            const string UrlEncodedReplacementChar = "%EF%BF%BD";
-            const string HighSurrogate = "\ud83f";
-            const string LowSurrogate = "\udffe";
-
-            yield return new object[] { "a", "a" };
-            yield return new object[] { HighSurrogate + LowSurrogate, "%F0%9F%BF%BE" };
-            yield return new object[] { HighSurrogate, UrlEncodedReplacementChar };
-            yield return new object[] { LowSurrogate, UrlEncodedReplacementChar };
-            yield return new object[] { LowSurrogate + HighSurrogate, UrlEncodedReplacementChar + UrlEncodedReplacementChar };
-            yield return new object[] { LowSurrogate + LowSurrogate, UrlEncodedReplacementChar + UrlEncodedReplacementChar };
-            yield return new object[] { HighSurrogate + HighSurrogate, UrlEncodedReplacementChar + UrlEncodedReplacementChar };
-        }
-
-        [Theory]
-        [MemberData(nameof(ReplacesStandaloneSurrogatesWithReplacementChar))]
-        public static void ReplacesStandaloneSurrogatesWithReplacementChar(string input, string expected)
-        {
-            const string Prefix = "scheme:";
-            Uri uri = new Uri(Prefix + input);
-            string actual = uri.AbsoluteUri.Substring(Prefix.Length);
-            Assert.Equal(expected, actual);
-        }
-    }
-}
diff --git a/src/libraries/System.Private.Uri/tests/FunctionalTests/System.Private.Uri.Functional.Tests.csproj b/src/libraries/System.Private.Uri/tests/FunctionalTests/System.Private.Uri.Functional.Tests.csproj
index ca63421c45fca..a568e2d1069f1 100644
--- a/src/libraries/System.Private.Uri/tests/FunctionalTests/System.Private.Uri.Functional.Tests.csproj
+++ b/src/libraries/System.Private.Uri/tests/FunctionalTests/System.Private.Uri.Functional.Tests.csproj
@@ -4,7 +4,6 @@
   </PropertyGroup>
   <ItemGroup>
     <Compile Include="AppxUriValue.cs" />
-    <Compile Include="EscapeUnescapeIriTests.cs" />
     <Compile Include="IdnCheckHostNameTest.cs" />
     <Compile Include="IdnDnsSafeHostTest.cs" />
     <Compile Include="IdnHostNameValidationTest.cs" />