diff --git a/benchmark/declaration-headers/README.md b/benchmark/declaration-headers/README.md
new file mode 100644
index 00000000..a52d86fc
--- /dev/null
+++ b/benchmark/declaration-headers/README.md
@@ -0,0 +1,55 @@
+# Declaration header generation benchmark
+
+Creates 300 PHP source files with 6,001 functions, each ordinary function
+having a default parameter. It prepares and converts the project, then measures
+six calls to the real declaration-header generator and discards the first as
+warm-up. The timed region includes rendering and writing headers; source
+preparation, conversion, and C++ compilation are excluded. This measures one
+compiler phase, not total build time.
+
+Run from a checkout with Composer development dependencies installed:
+
+```sh
+project_dir=$(mktemp -d)
+php benchmark/declaration-headers/run.php "$project_dir" > /tmp/declaration-candidate.log
+tail -1 /tmp/declaration-candidate.log
+```
+
+The script writes generated sources and build outputs into the supplied
+benchmark directory. Reuse the same directory for comparisons: absolute source
+paths contribute to header names and therefore the combined content digest.
+
+The baseline does not contain this new benchmark. From the candidate checkout,
+these commands create a separate baseline and run the same source in both. The
+shared vendor directory is supported by `phpunit/bootstrap.php`'s checkout-local
+source loader.
+
+```sh
+baseline_dir=$(mktemp -d)
+project_dir=$(mktemp -d)
+git archive b3898c32 | tar -x -C "$baseline_dir"
+ln -s "$(pwd)/vendor" "$baseline_dir/vendor"
+mkdir -p "$baseline_dir/benchmark/declaration-headers"
+cp benchmark/declaration-headers/run.php "$baseline_dir/benchmark/declaration-headers/run.php"
+php "$baseline_dir/benchmark/declaration-headers/run.php" "$project_dir" > /tmp/declaration-baseline.log
+php benchmark/declaration-headers/run.php "$project_dir" > /tmp/declaration-candidate.log
+tail -1 /tmp/declaration-baseline.log
+tail -1 /tmp/declaration-candidate.log
+```
+
+Compare median `ms` values and require identical `headers` counts and `sha256`
+digests. The digest covers the contents and names of every generated declaration
+header, including runtime and aggregate headers.
+
+## Sample result
+
+Linux ARM64 Docker, PHP 8.5.10 ZTS, CLI opcache disabled; baseline `b3898c32`:
+
+| Revision | Median (ms) | Headers |
+| --- | ---: | ---: |
+| Baseline | 88.001 | 302 |
+| Grouped functions | 13.087 | 302 |
+
+The measured phase is about 6.72x faster, with identical output digests. Raw
+samples are in `results-arm64.json`. This workload isolates function-heavy
+projects; class and constant declaration scans are not optimized by this change.
diff --git a/benchmark/declaration-headers/results-arm64.json b/benchmark/declaration-headers/results-arm64.json
new file mode 100644
index 00000000..358210c4
--- /dev/null
+++ b/benchmark/declaration-headers/results-arm64.json
@@ -0,0 +1,31 @@
+{
+  "environment": "Linux ARM64 Docker, PHP 8.5.10 ZTS, no CLI opcache",
+  "baseline_commit": "b3898c32",
+  "files": 300,
+  "functions": 6001,
+  "baseline": {
+    "ms": [
+      88.000792,
+      87.998334,
+      90.341888,
+      90.412138,
+      87.537832
+    ],
+    "headers": 302,
+    "sha256": "bfad6f04ca81c591734aeeaf223cc32ef7064768ca12faba8ac41ef053444f37",
+    "median_ms": 88.000792
+  },
+  "candidate": {
+    "ms": [
+      13.015777,
+      13.490487,
+      12.763525,
+      13.087318,
+      13.106777
+    ],
+    "headers": 302,
+    "sha256": "bfad6f04ca81c591734aeeaf223cc32ef7064768ca12faba8ac41ef053444f37",
+    "median_ms": 13.087318
+  },
+  "speedup": 6.7241272810823425
+}
diff --git a/benchmark/declaration-headers/run.php b/benchmark/declaration-headers/run.php
new file mode 100644
index 00000000..ee4ea44d
--- /dev/null
+++ b/benchmark/declaration-headers/run.php
@@ -0,0 +1,43 @@
+<?php
+require dirname(__DIR__, 2) . '/phpunit/bootstrap.php';
+if ($argc !== 2 || !is_dir($argv[1])) {
+    fwrite(STDERR, "Usage: php run.php <existing temporary project directory>\n");
+    exit(1);
+}
+$root = realpath($argv[1]);
+$files = [];
+for ($i = 0; $i < 300; ++$i) {
+    $source = "<?php\n";
+    for ($j = 0; $j < 20; ++$j) {
+        $source .= "function f_{$i}_{$j}(int \$value = 1): int { return \$value; }\n";
+    }
+    if ($i === 0) {
+        $source .= "function main(): void {}\n";
+    }
+    $file = "$root/file_$i.php";
+    file_put_contents($file, $source);
+    $files[] = $file;
+}
+$translator = $compiler = TypePhp\CompilerTest::create($root);
+$compiler->setTargetName('declaration_benchmark');
+(new ReflectionMethod($compiler, 'setBuildDir'))->invoke($compiler, "$root/build");
+$compiler->addFiles($files);
+foreach ($files as $file) {
+    $compiler->prepareFile($file);
+}
+$compiler->convert($files);
+$generate = new ReflectionMethod($compiler, 'genDeclarationHeaders');
+$samples = [];
+for ($i = 0; $i < 6; ++$i) {
+    $start = hrtime(true);
+    $generate->invoke($compiler, $files);
+    if ($i > 0) {
+        $samples[] = (hrtime(true) - $start) / 1e6;
+    }
+}
+$hashes = [];
+foreach (glob("$root/build/include/*decl.h") as $file) {
+    $hashes[basename($file)] = hash_file('sha256', $file);
+}
+ksort($hashes);
+echo "BENCHMARK_RESULT=" . json_encode(['ms' => $samples, 'headers' => count($hashes), 'sha256' => hash('sha256', json_encode($hashes))]) . "\n";
diff --git a/phpunit/src/Build/IncrementalDeclarationTest.php b/phpunit/src/Build/IncrementalDeclarationTest.php
index 0f646e0e..fedc6318 100644
--- a/phpunit/src/Build/IncrementalDeclarationTest.php
+++ b/phpunit/src/Build/IncrementalDeclarationTest.php
@@ -122,6 +122,42 @@ PHP);
         self::assertSame($this->provider, $symbols['constant:Incremental\\LIMIT']);
     }
 
+    public function testGroupedDeclarationsPreserveHelpersAndFilesWithoutFunctions(): void
+    {
+        file_put_contents($this->provider, <<<'PHP'
+<?php
+namespace Incremental;
+abstract class Base
+{
+    abstract public function amount(int $value = 5): int;
+}
+function answer(int $value = 42, string ...$labels): int { return $value; }
+PHP);
+        file_put_contents($this->consumer, "<?php\nfunction main(): int { return \\Incremental\\answer(); }\n");
+        file_put_contents($this->independent, "<?php\nconst MARKER = 1;\n");
+
+        $compiler = $this->convertProject();
+        foreach ([$this->provider, $this->consumer, $this->independent] as $file) {
+            $expected = $this->invoke($compiler, 'renderFunctionDeclarations', $file)
+                . $this->invoke($compiler, 'renderDataDeclarations', $file, false, false);
+            self::assertSame($expected, file_get_contents($compiler->getDeclarationHeaderFile($file)));
+        }
+        $provider = file_get_contents($compiler->getDeclarationHeaderFile($this->provider));
+        self::assertStringContainsString('php_incremental__answer_arg_0_default_value();', $provider);
+        self::assertStringContainsString('php_incremental__answer_arg_1_default_value();', $provider);
+        self::assertStringContainsString('php_incremental__base__amount_arg_0_default_value();', $provider);
+        self::assertStringNotContainsString('php_main(', $provider);
+        $independent = file_get_contents($compiler->getDeclarationHeaderFile($this->independent));
+        self::assertStringNotContainsString('php_incremental__answer', $independent);
+        self::assertStringNotContainsString('php_main(', $independent);
+        self::assertStringNotContainsString('_default_value', $independent);
+
+        $aggregate = $this->invoke($compiler, 'renderFunctionDeclarations');
+        self::assertStringContainsString('php_incremental__answer(', $aggregate);
+        self::assertStringContainsString('php_main(', $aggregate);
+        self::assertStringContainsString('php_incremental__base__amount_arg_0_default_value();', $aggregate);
+    }
+
     public function testSuperglobalsAreDeclaredInEachUsingSource(): void
     {
         file_put_contents($this->provider, <<<'PHP'
diff --git a/src/Generator/DefaultArgumentGenerator.php b/src/Generator/DefaultArgumentGenerator.php
index 89662068..357ac581 100644
--- a/src/Generator/DefaultArgumentGenerator.php
+++ b/src/Generator/DefaultArgumentGenerator.php
@@ -11,6 +11,7 @@ use TypePhp\Type;
 
 use TypePhp\Entity\ArgInfo;
 use TypePhp\Entity\ArrayInitPlan;
+use TypePhp\Entity\FunctionDef;
 
 trait DefaultArgumentGenerator
 {
@@ -43,13 +44,11 @@ trait DefaultArgumentGenerator
         return "do {\n" . $plan->init . $body . $plan->clean . "} while (0);\n";
     }
 
-    protected function genDefaultArgumentHelperDeclarations(?string $sourceFile = null): string
+    /** @param array<string, FunctionDef> $functions */
+    protected function genDefaultArgumentHelperDeclarations(array $functions): string
     {
         $code = '';
-        foreach ($this->symbols->functions() as $nativeName => $func) {
-            if ($sourceFile !== null && $func->sourceFile !== $sourceFile) {
-                continue;
-            }
+        foreach ($functions as $nativeName => $func) {
             foreach ($func->argInfoList as $argumentIndex => $argInfo) {
                 if (!$this->shouldGenerateDefaultArgumentHelper($argInfo)) {
                     continue;
diff --git a/src/Translator.php b/src/Translator.php
index 24bc5b06..b9e82f58 100644
--- a/src/Translator.php
+++ b/src/Translator.php
@@ -2841,8 +2841,15 @@ CODE;
         $this->writeFile($file, $this->renderFunctionDeclarations());
     }
 
-    protected function renderFunctionDeclarations(?string $sourceFile = null): string
+    /** @param array<string, FunctionDef>|null $functions */
+    protected function renderFunctionDeclarations(?string $sourceFile = null, ?array $functions = null): string
     {
+        $functions ??= $sourceFile === null
+            ? $this->symbols->functions()
+            : array_filter(
+                $this->symbols->functions(),
+                static fn (FunctionDef $function): bool => $function->sourceFile === $sourceFile,
+            );
         $code = '#pragma once' . PHP_EOL . PHP_EOL;
         $code .= '#include <phpx.h>' . PHP_EOL;
         $code .= '#include <typephp_helper.h>' . PHP_EOL;
@@ -2861,10 +2868,7 @@ CODE;
             $code .= $this->genLibraryApiMacro($this->targetName);
         }
         $importLibraries = [];
-        foreach ($this->symbols->functions() as $function) {
-            if ($sourceFile !== null && $function->sourceFile !== $sourceFile) {
-                continue;
-            }
+        foreach ($functions as $function) {
             if ($this->isImportedFunction($function)) {
                 $importLibraries[$function->importLibrary] = true;
             }
@@ -2873,12 +2877,9 @@ CODE;
             $code .= $this->genLibraryImportMacro($library);
         }
 
-        $code .= $this->genDefaultArgumentHelperDeclarations($sourceFile);
+        $code .= $this->genDefaultArgumentHelperDeclarations($functions);
 
-        foreach ($this->symbols->functions() as $name => $func) {
-            if ($sourceFile !== null && $func->sourceFile !== $sourceFile) {
-                continue;
-            }
+        foreach ($functions as $name => $func) {
             if ($func->abstractMethod) {
                 continue;
             }
@@ -3008,11 +3009,20 @@ CODE;
         $this->writeFile($runtimeHeader, '#pragma once' . PHP_EOL . PHP_EOL
             . $this->renderDataDeclarations(null, true, false)
             . $this->genNativeObjectForwardDeclarations());
+        $functionsBySource = null;
         foreach ($this->declarationHeaderFiles as $file => $header) {
             if (!$this->shouldRegeneratePhpFile($file)) {
                 continue;
             }
-            $code = $this->renderFunctionDeclarations($file);
+            // Group once, only if a header needs regeneration. Keep this local
+            // so later passes see symbols finalized by their own conversion.
+            if ($functionsBySource === null) {
+                $functionsBySource = [];
+                foreach ($this->symbols->functions() as $name => $function) {
+                    $functionsBySource[$function->sourceFile][$name] = $function;
+                }
+            }
+            $code = $this->renderFunctionDeclarations($file, $functionsBySource[$file] ?? []);
             $code .= $this->renderDataDeclarations($file, false, false);
             $this->writeFile(
                 $this->getIncludeDir() . '/' . $header,