perf(json): presize maps during FromJson decode

json/from_json.mbt

@@ -242,7 +242,7 @@ pub impl[V : FromJson] FromJson for Map[String, V] with fn from_json(json, path)
   guard json is Object(obj) else {
     decode_error(path, "Map::from_json: expected object")
   }
-  let res = Map([])
+  let res = Map([], capacity=obj.capacity())
   // As above: the test cannot fail, it just makes `key` assignable.
   guard! JsonPath::Key(path, key="") is (Key(_) as new_path)
   // Map::map_with_key

json/from_json_map_bench_test.mbt

@@ -0,0 +1,40 @@
+// Copyright 2026 International Digital Economy Academy
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+///|
+fn from_json_map_bench_input(size : Int) -> Json {
+  let object : Map[String, Json] = Map([], capacity=size)
+  for i in 0..<size {
+    object[i.to_string()] = Json::number(i.to_double())
+  }
+  Json::object(object)
+}
+
+///|
+test "bench Map::from_json entries=10000" (it : @bench.T) {
+  let input = from_json_map_bench_input(10000)
+  it.bench(fn() {
+    let decoded : Map[String, Int] = try! @json.from_json(input)
+    it.keep(decoded.length())
+  })
+}
+
+///|
+test "bench Map::from_json entries=100000" (it : @bench.T) {
+  let input = from_json_map_bench_input(100000)
+  it.bench(fn() {
+    let decoded : Map[String, Int] = try! @json.from_json(input)
+    it.keep(decoded.length())
+  })
+}