diff --git a/src/SourceGenerators/Uno.UI.SourceGenerators/XamlGenerator/XamlFileGenerator.cs b/src/SourceGenerators/Uno.UI.SourceGenerators/XamlGenerator/XamlFileGenerator.cs
index 8f4252dd8506..61a352e5f373 100644
--- a/src/SourceGenerators/Uno.UI.SourceGenerators/XamlGenerator/XamlFileGenerator.cs
+++ b/src/SourceGenerators/Uno.UI.SourceGenerators/XamlGenerator/XamlFileGenerator.cs
@@ -9,6 +9,7 @@
 using System.IO;
 using System.Linq;
 using System.Runtime.CompilerServices;
+using System.Runtime.InteropServices;
 using System.Text;
 using System.Text.RegularExpressions;
 using Microsoft.CodeAnalysis;
@@ -4564,26 +4565,99 @@ private string RewriteNamespaces(string xamlString)
 		{
 			foreach (var ns in _fileDefinition.Namespaces)
 			{
+				xamlString = ReplaceNamespace(xamlString, ns);
+			}
+
+			return xamlString;
+
+			// This is a performance-sensitive code. It used to be using Regex and was improved to avoid Regex.
+			// It might be possible to improve it further, but that seems to do the job for now.
+			// We can optimize further if future performance measures shows it being problematic.
+			// What this method does is:
+			// Given a xamlString like "muxc:SomeControl", converts it to Microsoft.UI.Xaml.Controls.SomeControl.
+			// Note that the given xamlString can be a more complex expression involving multiple namespaces.
+			static string ReplaceNamespace(string xamlString, NamespaceDeclaration ns)
+			{
+				// Note: The call xamlString.IndexOf($"{ns.Prefix}:", StringComparison.Ordinal) can be replaced with xamlString.IndexOf(ns.Prefix)
+				// followed by a separate check for ":" character. This will save a string allocation.
+				// But for now, we keep it as is. We can always revisit if it shows performance issues.
 				if (ns.Namespace.StartsWith("using:", StringComparison.Ordinal))
 				{
-					// Replace namespaces with their fully qualified namespace.
-					// Add global:: so that qualified paths can be expluded from binding
-					// path observation.
-					xamlString = Regex.Replace(
-						xamlString,
-						$@"(^|[^\w])({ns.Prefix}:)",
-						"$1global::" + ns.Namespace.TrimStart("using:") + ".");
+					while (xamlString.Length > ns.Prefix.Length)
+					{
+						var index = xamlString.IndexOf($"{ns.Prefix}:", StringComparison.Ordinal);
+						if (index == 0 || (index > 0 && !char.IsLetterOrDigit(xamlString[index - 1])))
+						{
+							var nsGlobalized = ns.Namespace.Replace("using:", "global::");
+
+							// following is the equivalent of "string.Concat(xamlString.Substring(0, index), nsGlobalized, ".", xamlString.Substring(index + ns.Prefix.Length + 1))"
+							// but in a way that allocates less memory.
+							var newLength = xamlString.Length + nsGlobalized.Length - ns.Prefix.Length;
+							xamlString = StringExtensions.Create(newLength, (xamlString, index, nsGlobalized, ns.Prefix), static (span, state) =>
+							{
+								var (xamlString, index, nsGlobalized, nsPrefix) = state;
+								var copiedLengthSoFar = 0;
+
+								xamlString.AsSpan().Slice(0, index).CopyTo(span.Slice(copiedLengthSoFar));
+								copiedLengthSoFar += index;
+
+								nsGlobalized.AsSpan().CopyTo(span.Slice(copiedLengthSoFar));
+								copiedLengthSoFar += nsGlobalized.Length;
+
+								span[copiedLengthSoFar] = '.';
+								copiedLengthSoFar++;
+
+								xamlString.AsSpan().Slice(index + nsPrefix.Length + 1).CopyTo(span.Slice(copiedLengthSoFar));
+							});
+
+						}
+						else
+						{
+							break;
+						}
+					}
+
+					return xamlString;
 				}
 				else if (ns.Namespace == XamlConstants.XamlXmlNamespace)
 				{
-					xamlString = Regex.Replace(
-						xamlString,
-						$@"(^|[^\w])({ns.Prefix}:)",
-						"$1global::System.");
+					while (xamlString.Length > ns.Prefix.Length)
+					{
+						var index = xamlString.IndexOf($"{ns.Prefix}:", StringComparison.Ordinal);
+						if (index == 0 || (index > 0 && !char.IsLetterOrDigit(xamlString[index - 1])))
+						{
+							const string nsGlobalized = "global::System.";
+
+							// following is the equivalent of "string.Concat(xamlString.Substring(0, index), nsGlobalized, xamlString.Substring(index + ns.Prefix.Length + 1))"
+							// but in a way that allocates less memory.
+							var newLength = xamlString.Length + nsGlobalized.Length - ns.Prefix.Length - 1;
+							xamlString = StringExtensions.Create(newLength, (xamlString, index, ns.Prefix), static (span, state) =>
+							{
+								var (xamlString, index, nsPrefix) = state;
+								var copiedLengthSoFar = 0;
+
+								xamlString.AsSpan().Slice(0, index).CopyTo(span.Slice(copiedLengthSoFar));
+								copiedLengthSoFar += index;
+
+								nsGlobalized.AsSpan().CopyTo(span.Slice(copiedLengthSoFar));
+								copiedLengthSoFar += nsGlobalized.Length;
+
+								xamlString.AsSpan().Slice(index + nsPrefix.Length + 1).CopyTo(span.Slice(copiedLengthSoFar));
+							});
+						}
+						else
+						{
+							break;
+						}
+					}
+
+					return xamlString;
+				}
+				else
+				{
+					return xamlString;
 				}
 			}
-
-			return xamlString;
 		}
 
 		private string GetDefaultBindMode() => _currentDefaultBindMode.Peek();
diff --git a/src/Uno.Foundation/Uno.Core.Extensions/Uno.Core.Extensions/StringExtensions.cs b/src/Uno.Foundation/Uno.Core.Extensions/Uno.Core.Extensions/StringExtensions.cs
index 24d3957c918b..485a19afcb66 100644
--- a/src/Uno.Foundation/Uno.Core.Extensions/Uno.Core.Extensions/StringExtensions.cs
+++ b/src/Uno.Foundation/Uno.Core.Extensions/Uno.Core.Extensions/StringExtensions.cs
@@ -27,6 +27,8 @@
 
 namespace Uno.Extensions
 {
+	public delegate void SpanAction<T, in TArg>(Span<T> span, TArg arg);
+
 	internal static partial class StringExtensions
 	{
 		// https://www.meziantou.net/split-a-string-into-lines-without-allocation.htm
@@ -153,5 +155,18 @@ public static string TrimEnd(this string source, string trimText)
 				return source;
 			}
 		}
+
+		public static string Create<TState>(int length, TState state, SpanAction<char, TState> action)
+		{
+			scoped Span<char> buffer;
+
+			if (length <= 128)
+				buffer = stackalloc char[128];
+			else
+				buffer = new char[length];
+
+			action(buffer.Slice(0, length), state);
+			return buffer.Slice(0, length).ToString();
+		}
 	}
 }