diff --git a/xapi-client/src/main/java/dev/learning/xapi/client/StatementHttpMessageWriter.java b/xapi-client/src/main/java/dev/learning/xapi/client/StatementHttpMessageWriter.java
index 2969fe99..71fc305e 100644
--- a/xapi-client/src/main/java/dev/learning/xapi/client/StatementHttpMessageWriter.java
+++ b/xapi-client/src/main/java/dev/learning/xapi/client/StatementHttpMessageWriter.java
@@ -188,10 +188,15 @@ private List<Object> getParts(Object object) {
    */
   private Stream<Attachment> getRealAttachments(Statement statement) {
 
+    Stream<Attachment> stream;
+
     // handle the rare scenario when a sub-statement has an attachment
-    var stream = statement.getObject() instanceof final SubStatement substatement
-        && substatement.getAttachments() != null ? substatement.getAttachments().stream()
-            : Stream.<Attachment>empty();
+    if (statement.getObject() instanceof final SubStatement substatement
+        && substatement.getAttachments() != null) {
+      stream = substatement.getAttachments().stream();
+    } else {
+      stream = Stream.empty();
+    }
 
     if (statement.getAttachments() != null) {
       stream = Stream.concat(stream, statement.getAttachments().stream());
diff --git a/xapi-model/src/main/java/dev/learning/xapi/model/Attachment.java b/xapi-model/src/main/java/dev/learning/xapi/model/Attachment.java
index dcca1fdf..0a831db3 100644
--- a/xapi-model/src/main/java/dev/learning/xapi/model/Attachment.java
+++ b/xapi-model/src/main/java/dev/learning/xapi/model/Attachment.java
@@ -210,11 +210,10 @@ private static String sha256Hex(byte[] data) {
       final var hash = digest.digest(data);
       final var hexString = new StringBuilder(2 * hash.length);
       for (final byte element : hash) {
-        final var hex = Integer.toHexString(0xff & element);
-        if (hex.length() == 1) {
-          hexString.append('0');
-        }
-        hexString.append(hex);
+        // Use bitwise operations to avoid string allocation per byte
+        final int value = 0xff & element;
+        hexString.append(Character.forDigit(value >>> 4, 16));
+        hexString.append(Character.forDigit(value & 0x0f, 16));
       }
       return hexString.toString();
     }
diff --git a/xapi-model/src/main/java/dev/learning/xapi/model/LanguageMap.java b/xapi-model/src/main/java/dev/learning/xapi/model/LanguageMap.java
index 5db5af6f..b94824a7 100644
--- a/xapi-model/src/main/java/dev/learning/xapi/model/LanguageMap.java
+++ b/xapi-model/src/main/java/dev/learning/xapi/model/LanguageMap.java
@@ -26,6 +26,11 @@ public class LanguageMap extends LinkedHashMap<Locale, String> {
 
   private static final long serialVersionUID = 7375610804995032187L;
 
+  /**
+   * Cached undefined locale instance to avoid repeated object creation.
+   */
+  private static final Locale UNDEFINED_LOCALE = new Locale("und");
+
   /**
    * Constructs an empty LanguageMap.
    */
@@ -49,7 +54,7 @@ public LanguageMap(Map<Locale, String> languageMap) {
    * @param value to be added with the undefined locale as a key
    */
   public void put(String value) {
-    this.put(new Locale("und"), value);
+    this.put(UNDEFINED_LOCALE, value);
   }
 
   /**
@@ -83,15 +88,15 @@ public String get(List<LanguageRange> languageRanges) {
     }
 
     // Otherwise return UND
-    final var und = get(new Locale("und"));
+    final var und = get(UNDEFINED_LOCALE);
     if (und != null) {
       return und;
     }
 
     // Otherwise return first
-    final var first = keySet().stream().findFirst();
-    if (first.isPresent()) {
-      return get(first.get());
+    final var iterator = keySet().iterator();
+    if (iterator.hasNext()) {
+      return get(iterator.next());
     }
 
     // Map must be empty
diff --git a/xapi-model/src/main/java/dev/learning/xapi/model/validation/internal/validators/MboxValidator.java b/xapi-model/src/main/java/dev/learning/xapi/model/validation/internal/validators/MboxValidator.java
index 907866d4..bb246a33 100644
--- a/xapi-model/src/main/java/dev/learning/xapi/model/validation/internal/validators/MboxValidator.java
+++ b/xapi-model/src/main/java/dev/learning/xapi/model/validation/internal/validators/MboxValidator.java
@@ -21,12 +21,14 @@ public class MboxValidator extends DisableableValidator<Mbox, String> {
 
   public static final String PREFIX = "mailto:";
 
-  EmailValidator emailValidator;
+  /**
+   * Cached email validator instance to avoid repeated object creation.
+   */
+  private static final EmailValidator EMAIL_VALIDATOR = new EmailValidator();
 
   @Override
   public void initialize(Mbox mbox) {
-
-    emailValidator = new EmailValidator();
+    // No initialization needed - using static validator
   }
 
   @Override
@@ -36,7 +38,7 @@ public boolean isValidIfEnabled(String value, ConstraintValidatorContext context
     }
 
     return value.startsWith(PREFIX)
-        && emailValidator.isValid(value.substring(PREFIX.length()), context);
+        && EMAIL_VALIDATOR.isValid(value.substring(PREFIX.length()), context);
   }
 
 }