diff --git a/.jules/bolt.md b/.jules/bolt.md
index 83cc604..e9f24f3 100644
--- a/.jules/bolt.md
+++ b/.jules/bolt.md
@@ -5,3 +5,7 @@
 ## 2024-05-24 - Loop Allocation Hot Paths
 **Learning:** Rendering directory entries with repeated string concatenation and list-based exclusion lookups creates avoidable allocation and lookup cost in large directories.
 **Action:** Use `StringBuilder` for entry rendering and a `Set` for excluded file names.
+
+## 2024-07-02 - String Escape Optimization
+**Learning:** Multiple passes of `.replace()` on strings cause significant performance degradation due to intermediate string allocations per replacement call.
+**Action:** Use a single-pass `StringBuilder` loop to avoid redundant string allocations when escaping HTML or doing multiple character replacements, after checking if an escape is even needed.
diff --git a/src/main/kotlin/html4tree/main.kt b/src/main/kotlin/html4tree/main.kt
index 2e2809f..6296a14 100644
--- a/src/main/kotlin/html4tree/main.kt
+++ b/src/main/kotlin/html4tree/main.kt
@@ -23,8 +23,9 @@ fun main(args: Array<String>)  = Html4tree().main(args)
 
 fun go(topDir: String, maxLevel: Int)  {
     require(topDir.isNotBlank())
-    val top_dir = File(topDir).canonicalFile
-    require(Files.isDirectory(top_dir.toPath(), LinkOption.NOFOLLOW_LINKS)) { "Top directory must be an existing non-symlink directory" }
+    val original_dir = File(topDir).absoluteFile
+    require(Files.isDirectory(original_dir.toPath(), LinkOption.NOFOLLOW_LINKS)) { "Top directory must be an existing non-symlink directory" }
+    val top_dir = original_dir.canonicalFile
 
     val ll = LinkedList()
 
@@ -49,12 +50,30 @@ fun go(topDir: String, maxLevel: Int)  {
 }
 
 fun String.escapeHtml(): String {
-    return this.replace("&", "&amp;")
-               .replace("<", "&lt;")
-               .replace(">", "&gt;")
-               .replace("\"", "&quot;")
-               .replace("'", "&#x27;")
-               .replace("`", "&#x60;")
+    var sb: java.lang.StringBuilder? = null
+    for (i in 0 until this.length) {
+        val c = this[i]
+        val escaped = when (c) {
+            '&' -> "&amp;"
+            '<' -> "&lt;"
+            '>' -> "&gt;"
+            '"' -> "&quot;"
+            '\'' -> "&#x27;"
+            '`' -> "&#x60;"
+            else -> null
+        }
+
+        if (escaped != null) {
+            if (sb == null) {
+                sb = java.lang.StringBuilder(this.length + 16)
+                sb.append(this, 0, i)
+            }
+            sb.append(escaped)
+        } else {
+            sb?.append(c)
+        }
+    }
+    return sb?.toString() ?: this
 }
 
 fun String.urlEncodePath(): String {
diff --git a/src/test/kotlin/html4tree/MainTest.kt b/src/test/kotlin/html4tree/MainTest.kt
index e8a3082..c36f7c8 100644
--- a/src/test/kotlin/html4tree/MainTest.kt
+++ b/src/test/kotlin/html4tree/MainTest.kt
@@ -39,6 +39,7 @@ class MainTest {
         assertEquals("&#x60;", "`".escapeHtml())
         assertEquals("&amp;&lt;&gt;&quot;&#x27;&#x60;", "&<>\"'`".escapeHtml())
         assertEquals("normal text", "normal text".escapeHtml())
+        assertEquals("mixed &amp; and &lt; text &gt; here", "mixed & and < text > here".escapeHtml())
     }
 
     @Test
@@ -85,6 +86,15 @@ class MainTest {
         }
     }
 
+    @Test
+    fun testGoRejectsNonExistentDir() {
+        val nonExistentDir = File(tempDir, "non_existent_dir_${System.currentTimeMillis()}")
+        assertFalse(nonExistentDir.exists(), "Test directory should not exist initially")
+        assertFailsWith<IllegalArgumentException> {
+            go(nonExistentDir.absolutePath, -1)
+        }
+    }
+
     @Test
     fun testGoEmptyDir() {
         go(tempDir.absolutePath, -1)