diff --git a/.jules/bolt.md b/.jules/bolt.md
index 39f32f6e..14ee10e1 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.
+
+## 2024-05-24 - Kotlin String Concatenation Bottleneck
+**Learning:** In Kotlin (as in Java), repeatedly using the `+=` operator for string concatenation within a loop leads to O(N^2) time complexity and massive memory reallocation overhead because strings are immutable. This becomes a major bottleneck when dynamically rendering HTML for directories containing a large number of files.
+**Action:** Always prefer `StringBuilder` when concatenating strings within a loop to maintain O(N) complexity and minimize memory allocations.
diff --git a/src/main/kotlin/html4tree/main.kt b/src/main/kotlin/html4tree/main.kt
index 50b2680d..4a940642 100644
--- a/src/main/kotlin/html4tree/main.kt
+++ b/src/main/kotlin/html4tree/main.kt
@@ -124,18 +124,19 @@ fun process_dir(curr_dir: File){
 """ 
 
     val index_middle = fun():String{ 
-        var l=""
+        val sb = StringBuilder()
 
         val dir_files: MutableList<File> = curr_dir.listFiles()?.toMutableList() ?: mutableListOf()
         dir_files.sortWith(compareBy ({it.name}) )
         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;" }} ${it.getName().escapeHtml()}</a></li>"""+"\n"
+              sb.append("""          <li><a style="display:block; width:100%" href="${if (isLinkedDirectory) { "./${it.getName().urlEncodePath()}/" } else { "./${it.getName().urlEncodePath()}" }}">${if (isLinkedDirectory) { "&#128193;" } else { "&rtrif;" }} ${it.getName().escapeHtml()}</a></li>""")
+              sb.append('\n')
            }
         }
 
-        return l;
+        return sb.toString();
      } 
 
    val index_bottom="""
diff --git a/src/main/kotlin/html4tree/util.kt b/src/main/kotlin/html4tree/util.kt
index 9a882d74..c8979c31 100644
--- a/src/main/kotlin/html4tree/util.kt
+++ b/src/main/kotlin/html4tree/util.kt
@@ -39,7 +39,7 @@ class LinkedList {
         if(l == null){
             return null
         } else {
-        	l.next = null
+            l.next = null
             return LinkedListEntry(l.data, l.level)
         }
     }