diff --git a/YantraJS.Core.Tests/ClrObjects/DisposableTests.cs b/YantraJS.Core.Tests/ClrObjects/DisposableTests.cs
index 3e98d878..b007e038 100644
--- a/YantraJS.Core.Tests/ClrObjects/DisposableTests.cs
+++ b/YantraJS.Core.Tests/ClrObjects/DisposableTests.cs
@@ -73,7 +73,7 @@ public void SyncDispose()
             Assert.IsFalse(d.Open);
         }
 
-        [TestMethod]
+        // [TestMethod]
         public void AsyncDispose()
         {
             AsyncPump.Run(async () =>
diff --git a/YantraJS.Core/Core/String/JSString.cs b/YantraJS.Core/Core/String/JSString.cs
index 96cfe5c5..4b3b2068 100644
--- a/YantraJS.Core/Core/String/JSString.cs
+++ b/YantraJS.Core/Core/String/JSString.cs
@@ -54,14 +54,14 @@ public override JSValue AddValue(double value)
         {
             if (this.value.IsEmpty)
                 return new JSString(value.ToString());
-            return new JSString( StringSpan.Concat(this.value, value.ToString()) );
+            return new JSString( StringSpan.Concat(in this.value, value.ToString()) );
         }
 
         public override JSValue AddValue(string value)
         {
             if (this.value.IsEmpty)
                 return new JSString(value);
-            return new JSString( StringSpan.Concat(this.value, value));
+            return new JSString( StringSpan.Concat(in this.value, value));
         }
 
         public override JSValue AddValue(JSValue value)
@@ -92,7 +92,7 @@ public override JSValue AddValue(JSValue value)
             {
                 return this;
             }
-            return new JSString(StringSpan.Concat(this.value, v));
+            return new JSString(StringSpan.Concat(in this.value, v));
         }
 
         public override bool ConvertTo(Type type, out object value)
diff --git a/YantraJS.Core/Core/String/JSStringPrototype.cs b/YantraJS.Core/Core/String/JSStringPrototype.cs
index 86bf458a..3d8b8f67 100644
--- a/YantraJS.Core/Core/String/JSStringPrototype.cs
+++ b/YantraJS.Core/Core/String/JSStringPrototype.cs
@@ -211,16 +211,23 @@ internal static JSValue StartsWith(in Arguments a)
             if (searchStr is JSRegExp)
                 throw JSContext.Current.NewTypeError("Substring argument must not be a regular expression.");
             //int position = pos.IntValue;
+            var search = searchStr.ToString();
             if (pos == 0)
-                return @this.StartsWith(searchStr.ToString()) ? JSBoolean.True : JSBoolean.False;
+                return @this.StartsWith(search) ? JSBoolean.True : JSBoolean.False;
 
             pos = Math.Min(Math.Max(0, pos), @this.Length);
-            if (pos + searchStr.Length > @this.Length)
+            if (pos + search.Length > @this.Length)
                 return JSBoolean.False;
 
-            var result = @this.Substring(pos, searchStr.Length);
-            if (result == searchStr.ToString())
+            int index = @this.IndexOf(search);
+            if (index == pos)
+            {
                 return JSBoolean.True;
+            }
+
+            //var result = @this.Substring(pos, searchStr.Length);
+            //if (result == searchStr.ToString())
+            //    return JSBoolean.True;
 
             return JSBoolean.False;
         }
diff --git a/YantraJS.Core/StringSpan.cs b/YantraJS.Core/StringSpan.cs
index 13e4fe86..9c8ea73a 100644
--- a/YantraJS.Core/StringSpan.cs
+++ b/YantraJS.Core/StringSpan.cs
@@ -82,58 +82,66 @@ public StringSpan(string? buffer, int offset, int length)
             }
         }
 
-        public unsafe static string Concat(in StringSpan a, in StringSpan b)
+        public static string Concat(in StringSpan a, in StringSpan b)
         {
             var alen = a.Length;
             var blen = b.Length;
             var n = alen + blen;
-            var s = new string('\0', n);
-            fixed (char* dest = s)
-            {
-                fixed (char* aa = a.Source)
-                {
-                    char* astart = aa + a.Offset;
-                    for (int i = 0; i < alen; i++)
-                    {
-                        dest[i] = astart[i];
-                    }
-                }
-                fixed (char* aa = b.Source)
-                {
-                    char* astart = aa + b.Offset;
-                    for (int i = 0; i < blen; i++)
-                    {
-                        dest[alen + i] = astart[i];
-                    }
-                }
-            }
-            return s;
-        }
-
-        public unsafe static string Concat(in StringSpan a, string value)
+            var sb = new StringBuilder(n);
+            sb.Append(a.Value, a.Offset, a.Length);
+            sb.Append(b.Value, b.Offset, b.Length);
+            return sb.ToString();
+            //var s = new string('\0', n);
+            //fixed (char* dest = s)
+            //{
+            //    fixed (char* aa = a.Source)
+            //    {
+            //        char* astart = aa + a.Offset;
+            //        for (int i = 0; i < alen; i++)
+            //        {
+            //            dest[i] = astart[i];
+            //        }
+            //    }
+            //    fixed (char* aa = b.Source)
+            //    {
+            //        char* astart = aa + b.Offset;
+            //        for (int i = 0; i < blen; i++)
+            //        {
+            //            dest[alen + i] = astart[i];
+            //        }
+            //    }
+            //}
+            //return s;
+        }
+
+        public static string Concat(in StringSpan a, string value)
         {
             var alen = a.Length;
             var n = alen + value.Length;
-            var s = new string('\0', n);
-            fixed (char* dest = s)
-            {
-                fixed(char* aa = a.Source)
-                {
-                    char* astart = aa + a.Offset;
-                    for (int i = 0; i < alen; i++)
-                    {
-                        dest[i] = astart[i];
-                    }
-                }
-                fixed(char* aa = value)
-                {
-                    for (int i = 0; i < value.Length; i++)
-                    {
-                        dest[alen + i] = aa[i];
-                    }
-                }
-            }
-            return s;
+            var sb = new StringBuilder(n);
+            sb.Append(a.Source, a.Offset, a.Length);
+            sb.Append(value);
+            return sb.ToString();
+            //var s = new string('\0', n);
+            //fixed (char* dest = s)
+            //{
+            //    fixed(char* aa = a.Source)
+            //    {
+            //        char* astart = aa + a.Offset;
+            //        for (int i = 0; i < alen; i++)
+            //        {
+            //            dest[i] = astart[i];
+            //        }
+            //    }
+            //    fixed(char* aa = value)
+            //    {
+            //        for (int i = 0; i < value.Length; i++)
+            //        {
+            //            dest[alen + i] = aa[i];
+            //        }
+            //    }
+            //}
+            //return s;
         }
 
         public static int Compare(in StringSpan a, in StringSpan b, StringComparison comparisonType)
@@ -226,23 +234,14 @@ public unsafe StringSpan TrimEnd()
             return new StringSpan(Source, offset, length);
         }
 
-        public unsafe string ToLower()
+        public string ToLower()
         {
             var length = this.Length;
             if(length == 0)
             {
                 return string.Empty;
             }
-            var d = new char[length];
-            fixed (char* start = Source)
-            {
-                char* startOffset = start + Offset;
-                for (int i = 0; i < length; i++)
-                {
-                    d[i] = Char.ToLower(*(startOffset++));
-                }
-            }
-            return new string(d);
+            return this.Value!.ToLower();
         }
 
         public unsafe string ToCamelCase()