diff --git a/src/libraries/System.Private.CoreLib/src/System/Text/StringBuilder.cs b/src/libraries/System.Private.CoreLib/src/System/Text/StringBuilder.cs
index 8a8d6a93b57948..a74a10af7ea8dd 100644
--- a/src/libraries/System.Private.CoreLib/src/System/Text/StringBuilder.cs
+++ b/src/libraries/System.Private.CoreLib/src/System/Text/StringBuilder.cs
@@ -981,7 +981,9 @@ public void CopyTo(int sourceIndex, Span<char> destination, int count)
         /// <param name="index">The index to insert in this builder.</param>
         /// <param name="value">The string to insert.</param>
         /// <param name="count">The number of times to insert the string.</param>
-        public StringBuilder Insert(int index, string? value, int count)
+        public StringBuilder Insert(int index, string? value, int count) => Insert(index, value.AsSpan(), count);
+
+        private StringBuilder Insert(int index, ReadOnlySpan<char> value, int count)
         {
             if (count < 0)
             {
@@ -994,7 +996,7 @@ public StringBuilder Insert(int index, string? value, int count)
                 throw new ArgumentOutOfRangeException(nameof(index), SR.ArgumentOutOfRange_Index);
             }
 
-            if (string.IsNullOrEmpty(value) || count == 0)
+            if (value.IsEmpty || count == 0)
             {
                 return this;
             }
@@ -1012,7 +1014,7 @@ public StringBuilder Insert(int index, string? value, int count)
 
             while (count > 0)
             {
-                ReplaceInPlaceAtChunk(ref chunk!, ref indexInChunk, ref value.GetRawStringData(), value.Length);
+                ReplaceInPlaceAtChunk(ref chunk!, ref indexInChunk, ref MemoryMarshal.GetReference(value), value.Length);
                 --count;
             }
 
@@ -1290,17 +1292,25 @@ public StringBuilder Insert(int index, string? value)
             return this;
         }
 
-        public StringBuilder Insert(int index, bool value) => Insert(index, value.ToString(), 1);
+#pragma warning disable CA1830 // Prefer strongly-typed Append and Insert method overloads on StringBuilder. No need to fix for the builder itself
+        // bool does not implement ISpanFormattable but its ToString override returns cached strings.
+        public StringBuilder Insert(int index, bool value) => Insert(index, value.ToString().AsSpan(), 1);
+#pragma warning restore CA1830
 
         [CLSCompliant(false)]
-        public StringBuilder Insert(int index, sbyte value) => Insert(index, value.ToString(), 1);
+        public StringBuilder Insert(int index, sbyte value) => InsertSpanFormattable(index, value);
 
-        public StringBuilder Insert(int index, byte value) => Insert(index, value.ToString(), 1);
+        public StringBuilder Insert(int index, byte value) => InsertSpanFormattable(index, value);
 
-        public StringBuilder Insert(int index, short value) => Insert(index, value.ToString(), 1);
+        public StringBuilder Insert(int index, short value) => InsertSpanFormattable(index, value);
 
         public StringBuilder Insert(int index, char value)
         {
+            if ((uint)index > (uint)Length)
+            {
+                throw new ArgumentOutOfRangeException(nameof(index), SR.ArgumentOutOfRange_Index);
+            }
+
             Insert(index, ref value, 1);
             return this;
         }
@@ -1314,7 +1324,7 @@ public StringBuilder Insert(int index, char[]? value)
 
             if (value != null)
             {
-                Insert(index, value, 0, value.Length);
+                Insert(index, ref MemoryMarshal.GetArrayDataReference(value), value.Length);
             }
             return this;
         }
@@ -1359,24 +1369,24 @@ public StringBuilder Insert(int index, char[]? value, int startIndex, int charCo
             return this;
         }
 
-        public StringBuilder Insert(int index, int value) => Insert(index, value.ToString(), 1);
+        public StringBuilder Insert(int index, int value) => InsertSpanFormattable(index, value);
 
-        public StringBuilder Insert(int index, long value) => Insert(index, value.ToString(), 1);
+        public StringBuilder Insert(int index, long value) => InsertSpanFormattable(index, value);
 
-        public StringBuilder Insert(int index, float value) => Insert(index, value.ToString(), 1);
+        public StringBuilder Insert(int index, float value) => InsertSpanFormattable(index, value);
 
-        public StringBuilder Insert(int index, double value) => Insert(index, value.ToString(), 1);
+        public StringBuilder Insert(int index, double value) => InsertSpanFormattable(index, value);
 
-        public StringBuilder Insert(int index, decimal value) => Insert(index, value.ToString(), 1);
+        public StringBuilder Insert(int index, decimal value) => InsertSpanFormattable(index, value);
 
         [CLSCompliant(false)]
-        public StringBuilder Insert(int index, ushort value) => Insert(index, value.ToString(), 1);
+        public StringBuilder Insert(int index, ushort value) => InsertSpanFormattable(index, value);
 
         [CLSCompliant(false)]
-        public StringBuilder Insert(int index, uint value) => Insert(index, value.ToString(), 1);
+        public StringBuilder Insert(int index, uint value) => InsertSpanFormattable(index, value);
 
         [CLSCompliant(false)]
-        public StringBuilder Insert(int index, ulong value) => Insert(index, value.ToString(), 1);
+        public StringBuilder Insert(int index, ulong value) => InsertSpanFormattable(index, value);
 
         public StringBuilder Insert(int index, object? value) => (value == null) ? this : Insert(index, value.ToString(), 1);
 
@@ -1395,6 +1405,21 @@ public StringBuilder Insert(int index, ReadOnlySpan<char> value)
             return this;
         }
 
+        private StringBuilder InsertSpanFormattable<T>(int index, T value) where T : ISpanFormattable
+        {
+            Debug.Assert(typeof(T).Assembly.Equals(typeof(object).Assembly), "Implementation trusts the results of TryFormat because T is expected to be something known");
+
+            Span<char> buffer = stackalloc char[256];
+            if (value.TryFormat(buffer, out int charsWritten, format: default, provider: null))
+            {
+                // We don't use Insert(int, ReadOnlySpan<char>) for exception compatibility;
+                // we want exceeding the maximum capacity to throw an OutOfMemoryException.
+                return Insert(index, buffer.Slice(0, charsWritten), 1);
+            }
+
+            return Insert(index, value.ToString(), 1);
+        }
+
         public StringBuilder AppendFormat(string format, object? arg0) => AppendFormatHelper(null, format, new ParamsArray(arg0));
 
         public StringBuilder AppendFormat(string format, object? arg0, object? arg1) => AppendFormatHelper(null, format, new ParamsArray(arg0, arg1));
@@ -2066,10 +2091,7 @@ private void AppendWithExpansion(ref char value, int valueCount)
         /// <param name="valueCount">The number of characters in the buffer.</param>
         private void Insert(int index, ref char value, int valueCount)
         {
-            if ((uint)index > (uint)Length)
-            {
-                throw new ArgumentOutOfRangeException(nameof(index), SR.ArgumentOutOfRange_Index);
-            }
+            Debug.Assert((uint)index <= (uint)Length, "Callers should check that index is a legal value.");
 
             if (valueCount > 0)
             {