diff --git a/.gitignore b/.gitignore
index 54a5f5de..943bdf17 100644
--- a/.gitignore
+++ b/.gitignore
@@ -4,4 +4,5 @@
 # Ignore Gradle build output directory
 build
 
-.idea
\ No newline at end of file
+.idea
+perf_test/
diff --git a/.jules/bolt.md b/.jules/bolt.md
index 39f32f6e..9e53d6a3 100644
--- a/.jules/bolt.md
+++ b/.jules/bolt.md
@@ -1,3 +1,7 @@
-## 2024-06-21 - Regex Compilation in Loops
-**Learning:** In Kotlin, compiling regular expressions (`.toRegex()`) inside a loop over files is a significant O(N * M) performance bottleneck when processing ignore files (N files * M rules).
-**Action:** Always map string rules to compiled `Regex` objects outside of the file iteration loop (O(M) compilation) to avoid unnecessary regex re-compilations.
+## 2024-05-18 - StringBuilder over String Concatenation in Kotlin
+**Learning:** In Kotlin (and Java), using `+=` for string concatenation within a loop has an O(N²) time complexity due to the immutability of `String`, which requires creating a new `String` and copying the contents on every iteration. This can become a significant bottleneck when processing large collections (like thousands of directory files).
+**Action:** Always prefer using a `StringBuilder` (and ideally initializing it with an estimated capacity) for string accumulation inside loops to maintain O(N) performance and avoid unnecessary memory allocations.
+
+## 2024-05-18 - StringBuilder over String Concatenation in Kotlin
+**Learning:** In Kotlin (and Java), using `+=` for string concatenation within a loop has an O(N²) time complexity due to the immutability of `String`, which requires creating a new `String` and copying the contents on every iteration. This can become a significant bottleneck when processing large collections (like thousands of directory files).
+**Action:** Always prefer using a `StringBuilder` (and ideally initializing it with an estimated capacity) for string accumulation inside loops to maintain O(N) performance and avoid unnecessary memory allocations.
diff --git a/src/main/kotlin/html4tree/main.kt b/src/main/kotlin/html4tree/main.kt
index 50b2680d..02cf4daa 100644
--- a/src/main/kotlin/html4tree/main.kt
+++ b/src/main/kotlin/html4tree/main.kt
@@ -124,18 +124,22 @@ fun process_dir(curr_dir: File){
 """ 
 
     val index_middle = fun():String{ 
-        var l=""
-
         val dir_files: MutableList<File> = curr_dir.listFiles()?.toMutableList() ?: mutableListOf()
         dir_files.sortWith(compareBy ({it.name}) )
+
+        // ⚡ Bolt: 문자열 결합 성능 최적화 (O(N^2) -> O(N))
+        // StringBuilder를 사용하여 리스트 항목들을 하나로 합칩니다.
+        // dir_files의 크기를 기반으로 초기 용량을 설정하여 재할당 오버헤드를 방지합니다.
+        val l = StringBuilder(dir_files.size * 250)
+
         dir_files.forEach {
            val isLinkedDirectory = it.isDirectory() && !java.nio.file.Files.isSymbolicLink(it.toPath())
            if((it.getName() !in exclude) && (isLinkedDirectory || !it.isDirectory())) {
-              l += """          <li><a style="display:block; width:100%" href="${if (isLinkedDirectory) { "./${it.getName().urlEncodePath()}/" } else { "./${it.getName().urlEncodePath()}" }}">${if (isLinkedDirectory) { "&#128193;" } else { "&rtrif;" }} ${it.getName().escapeHtml()}</a></li>"""+"\n"
+              l.append("""          <li><a style="display:block; width:100%" href="${if (isLinkedDirectory) { "./${it.getName().urlEncodePath()}/" } else { "./${it.getName().urlEncodePath()}" }}">${if (isLinkedDirectory) { "&#128193;" } else { "&rtrif;" }} ${it.getName().escapeHtml()}</a></li>"""+"\n")
            }
         }
 
-        return l;
+        return l.toString()
      } 
 
    val index_bottom="""
diff --git a/src/test/kotlin/html4tree/HelpTest.kt b/src/test/kotlin/html4tree/HelpTest.kt
new file mode 100644
index 00000000..20976e8b
--- /dev/null
+++ b/src/test/kotlin/html4tree/HelpTest.kt
@@ -0,0 +1,23 @@
+package html4tree
+
+import org.junit.Test
+import kotlin.test.assertEquals
+import java.io.ByteArrayOutputStream
+import java.io.PrintStream
+
+class HelpTest {
+    @Test
+    fun testHelp() {
+        val originalOut = System.out
+        val baos = ByteArrayOutputStream()
+        val ps = PrintStream(baos)
+        System.setOut(ps)
+
+        try {
+            help()
+            assertEquals("ERROR: help has not been written yet!\n", baos.toString().replace("\r\n", "\n"))
+        } finally {
+            System.setOut(originalOut)
+        }
+    }
+}