diff --git a/.jules/bolt.md b/.jules/bolt.md
new file mode 100644
index 00000000..0eed87b3
--- /dev/null
+++ b/.jules/bolt.md
@@ -0,0 +1,3 @@
+## 2026-06-28 - Fast Hex String Conversion
+**Learning:** Using `String.format("%02x", b)` inside a loop for converting byte arrays to hex strings is a significant performance bottleneck due to string parsing overhead per byte.
+**Action:** Use `java.util.HexFormat.of().formatHex(bytes)` instead for a massive performance improvement when converting byte arrays to hex strings in Java 17+.
diff --git a/CHANGELOG.md b/CHANGELOG.md
new file mode 100644
index 00000000..78e15496
--- /dev/null
+++ b/CHANGELOG.md
@@ -0,0 +1,5 @@
+# 변경 사항
+
+## [Unreleased]
+### 성능 개선 (Performance)
+- `DefaultDocumentConversionService` 내부의 `String.format` 루프를 사용하던 비효율적인 Hex 문자열 변환 로직을 `java.util.HexFormat`을 사용하도록 변경하여 성능 크게 최적화
diff --git a/src/main/java/com/clearfolio/viewer/service/DefaultDocumentConversionService.java b/src/main/java/com/clearfolio/viewer/service/DefaultDocumentConversionService.java
index 9c595937..221fb175 100644
--- a/src/main/java/com/clearfolio/viewer/service/DefaultDocumentConversionService.java
+++ b/src/main/java/com/clearfolio/viewer/service/DefaultDocumentConversionService.java
@@ -121,12 +121,8 @@ private String contentHash(MultipartFile file) {
             }
 
             byte[] raw = digest.digest();
-            StringBuilder hex = new StringBuilder(raw.length * 2);
-            for (byte b : raw) {
-                hex.append(String.format("%02x", b));
-            }
-
-            return hex.toString();
+            // HexFormat is faster than String.format
+            return java.util.HexFormat.of().formatHex(raw);
         } catch (NoSuchAlgorithmException ex) {
             throw new IllegalStateException("SHA-256 digest unavailable", ex);
         } catch (IOException ex) {