perf: bulk-append unescaped character runs in JSON parseString

Replace per-character StringBuilder.Append(char) calls with
StringBuilder.Append(string, start, length) for consecutive runs of
unescaped characters in the JSON string parser.

For strings with no escape sequences (the common case in real-world
JSON), this reduces the number of Append calls from O(n) to O(1),
which should meaningfully speed up JsonValue.Parse on workloads
with many string values.

The approach mirrors what JsonStringEncodeTo already does for
serialisation: track a chunk start position and flush accumulated
characters as a bulk substring when an escape or closing quote is hit.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: github-actions[bot]

RELEASE_NOTES.md

@@ -1,5 +1,9 @@
 # Release Notes
 
+## 8.1.4 - Mar 30 2026
+
+- Performance: `JsonValue.Parse` now copies unescaped string runs in bulk instead of appending character-by-character, reducing `StringBuilder.Append` calls for strings with few or no escape sequences
+
 ## 8.1.3 - Mar 23 2026
 
 - Fix JSON `/* ... */` comment parser: `*` or `/` characters inside the comment body no longer cause premature termination and parse failure

src/FSharp.Data.Json.Core/JsonValue.fs

@@ -315,8 +315,18 @@ type private JsonParser(jsonText: string) =
         ensure (i < s.Length && s.[i] = '"')
         i <- i + 1
 
+        // Track start of current unescaped run; flush as a bulk chunk when an escape or end is hit.
+        // This avoids per-character StringBuilder.Append calls for strings with few/no escapes,
+        // which is the common case in real-world JSON.
+        let mutable chunkStart = i
+
+        let inline flushChunk upTo =
+            if upTo > chunkStart then
+                buf.Append(s, chunkStart, upTo - chunkStart) |> ignore
+
         while i < s.Length && s.[i] <> '"' do
             if s.[i] = '\\' then
+                flushChunk i
                 ensure (i + 1 < s.Length)
 
                 match s.[i + 1] with
@@ -371,10 +381,12 @@ type private JsonParser(jsonText: string) =
                 | _ -> throw ()
 
                 i <- i + 2 // skip past \ and next char
+                chunkStart <- i
             else
-                buf.Append(s.[i]) |> ignore
                 i <- i + 1
 
+        // Flush any remaining unescaped characters
+        flushChunk i
         ensure (i < s.Length && s.[i] = '"')
         i <- i + 1
         let str = buf.ToString()