diff --git a/src/FSharp.Data.Html.Core/HtmlCssSelectors.fs b/src/FSharp.Data.Html.Core/HtmlCssSelectors.fs
index ff8e9cead..15749d7f9 100644
--- a/src/FSharp.Data.Html.Core/HtmlCssSelectors.fs
+++ b/src/FSharp.Data.Html.Core/HtmlCssSelectors.fs
@@ -59,7 +59,7 @@ module internal HtmlCssSelectors =
 
             (isHexadecimalDigit || c = '\n' || c = '\f' || c = '\r') |> not
 
-        let rec readString acc =
+        let rec readStringIntoSb (acc: System.Text.StringBuilder) =
             function
             | c :: t when
                 Char.IsLetterOrDigit(c)
@@ -68,20 +68,28 @@ module internal HtmlCssSelectors =
                 || c.Equals('+')
                 || c.Equals('/')
                 ->
-                readString (acc + (c.ToString())) t
+                acc.Append(c) |> ignore
+                readStringIntoSb acc t
             | '\'' :: t ->
                 if inQuotes then
                     inQuotes <- false
-                    acc, t
+                    acc.ToString(), t
                 else
                     inQuotes <- true
-                    readString acc t
-            | '\\' :: c :: t when isCharacterEscapable c -> readString (acc + (c.ToString())) t
-            | c :: t when inQuotes -> readString (acc + (c.ToString())) t
-            | c :: t -> acc, c :: t
-            | [] -> acc, []
+                    readStringIntoSb acc t
+            | '\\' :: c :: t when isCharacterEscapable c ->
+                acc.Append(c) |> ignore
+                readStringIntoSb acc t
+            | c :: t when inQuotes ->
+                acc.Append(c) |> ignore
+                readStringIntoSb acc t
+            | c :: t -> acc.ToString(), c :: t
+            | [] -> acc.ToString(), []
             | c -> failwithf "Invalid css selector syntax at: %s" (new String(Array.ofList c))
 
+        let readString (initial: string) chars =
+            readStringIntoSb (System.Text.StringBuilder(initial)) chars
+
         let (|StartsWith|_|) (s: string) (items: char list) =
             let candidates = s.ToCharArray()
 
diff --git a/src/FSharp.Data.Json.Core/JsonValue.fs b/src/FSharp.Data.Json.Core/JsonValue.fs
index 21be4f008..0ae390387 100644
--- a/src/FSharp.Data.Json.Core/JsonValue.fs
+++ b/src/FSharp.Data.Json.Core/JsonValue.fs
@@ -165,9 +165,13 @@ type JsonValue =
                     || c = '\\'
 
                 if needsEscaping then
-                    // Write all accumulated unescaped characters in one operation using Substring
+                    // Write all accumulated unescaped characters in one operation
                     if i > lastWritePos then
+#if NETSTANDARD2_0
                         w.Write(value.Substring(lastWritePos, i - lastWritePos))
+#else
+                        w.Write(value.AsSpan(lastWritePos, i - lastWritePos))
+#endif
 
                     // Write the escaped character
                     if ci >= 0 && ci <= 7 || ci = 11 || ci >= 14 && ci <= 31 then
@@ -187,7 +191,11 @@ type JsonValue =
 
             // Write any remaining unescaped characters
             if lastWritePos < value.Length then
+#if NETSTANDARD2_0
                 w.Write(value.Substring(lastWritePos))
+#else
+                w.Write(value.AsSpan(lastWritePos))
+#endif
 
     /// <summary>Serializes this JsonValue to a string with the specified formatting options.</summary>
     /// <param name="saveOptions">Controls formatting: indented, compact, or compact with spaces.</param>