diff --git a/.jules/bolt.md b/.jules/bolt.md
index 341c7c9..78b53ea 100644
--- a/.jules/bolt.md
+++ b/.jules/bolt.md
@@ -67,3 +67,6 @@
 ## 2025-02-12 - [Fast Path Execution in Directory Traversal and Log Parsing]
 **Learning:** Checking for string existence (`if "silence_" not in stderr`) before invoking regex matchers provides significant speed improvements when parsing large blocks of text. Similarly, moving expensive I/O operations like `os.path.realpath` inside conditional blocks prevents redundant disk access when configuration (like path exclusions) isn't utilized.
 **Action:** When working on large text processing or disk operations, verify if early exit conditions or conditional execution can bypass the expensive system or library calls.
+## 2026-06-25 - [DOM Query and Parsing Optimization in Event Listeners]
+**Learning:** Querying the DOM (e.g., `document.querySelectorAll`) and parsing attributes (e.g., `Number.parseInt(btn.dataset.bytes)`) inside frequent event listeners like `input` introduces unnecessary repeated processing overhead on every keystroke.
+**Action:** Always precompute static values and cache NodeLists/Elements outside of frequent event handlers to improve client-side UI responsiveness.
diff --git a/saas_web.py b/saas_web.py
index 3a7b035..87d6aac 100644
--- a/saas_web.py
+++ b/saas_web.py
@@ -239,19 +239,25 @@ async def add_security_headers(request: Request, call_next):
                 preview.innerText = 'Selected file size: ' + text;
             }
 
+            // Optimize: Cache DOM references and precompute button values outside the input event listener.
+            // This prevents repeating O(N) DOM traversal and string parsing on every keystroke.
+            const targetBytesPreview = document.getElementById('target_bytes_preview');
+            const targetBytesButtons = Array.from(document.querySelectorAll('#preset_buttons_container .preset-btn')).map(btn => ({
+                el: btn,
+                val: Number.parseInt(btn.dataset.bytes, 10)
+            }));
+
             document.getElementById('target_bytes').addEventListener('input', function(e) {
                 const val = parseInt(this.value, 10);
-                const preview = document.getElementById('target_bytes_preview');
+                const preview = targetBytesPreview;
                 this.setCustomValidity('');
                 this.removeAttribute('aria-invalid');
                 preview.style.color = '#1e7e34';
 
-                const buttons = document.querySelectorAll('#preset_buttons_container .preset-btn');
-                buttons.forEach(btn => {
-                    const presetValue = Number.parseInt(btn.dataset.bytes, 10);
-                    btn.setAttribute(
+                targetBytesButtons.forEach(btn => {
+                    btn.el.setAttribute(
                         'aria-pressed',
-                        !e.isTrusted && presetValue === val ? 'true' : 'false'
+                        !e.isTrusted && btn.val === val ? 'true' : 'false'
                     );
                 });
 
@@ -266,19 +272,25 @@ async def add_security_headers(request: Request, call_next):
 
             });
 
+            // Optimize: Cache DOM references and precompute button values outside the input event listener.
+            // This prevents repeating O(N) DOM traversal and string parsing on every keystroke.
+            const batchTargetBytesPreview = document.getElementById('batch_target_bytes_preview');
+            const batchTargetBytesButtons = Array.from(document.querySelectorAll('#batch_preset_buttons_container .preset-btn')).map(btn => ({
+                el: btn,
+                val: Number.parseInt(btn.dataset.bytes, 10)
+            }));
+
             document.getElementById('batch_target_bytes').addEventListener('input', function(e) {
                 const val = parseInt(this.value, 10);
-                const preview = document.getElementById('batch_target_bytes_preview');
+                const preview = batchTargetBytesPreview;
                 this.setCustomValidity('');
                 this.removeAttribute('aria-invalid');
                 preview.style.color = '#1e7e34';
 
-                const buttons = document.querySelectorAll('#batch_preset_buttons_container .preset-btn');
-                buttons.forEach(btn => {
-                    const presetValue = Number.parseInt(btn.dataset.bytes, 10);
-                    btn.setAttribute(
+                batchTargetBytesButtons.forEach(btn => {
+                    btn.el.setAttribute(
                         'aria-pressed',
-                        !e.isTrusted && presetValue === val ? 'true' : 'false'
+                        !e.isTrusted && btn.val === val ? 'true' : 'false'
                     );
                 });
 
diff --git a/tests/test_saas_web.py b/tests/test_saas_web.py
index 57b879d..ef92bc1 100644
--- a/tests/test_saas_web.py
+++ b/tests/test_saas_web.py
@@ -331,8 +331,8 @@ def test_get_ui_includes_preset_buttons(self):
         self.assertIn('aria-pressed="false"', html)
         self.assertIn('role="group" aria-label="Preset target sizes"', html)
         self.assertNotIn('onclick="setTargetBytes(', html)
-        self.assertIn("const presetValue = Number.parseInt(btn.dataset.bytes, 10);", html)
-        self.assertIn("!e.isTrusted && presetValue === val", html)
+        self.assertIn("val: Number.parseInt(btn.dataset.bytes, 10)", html)
+        self.assertIn("!e.isTrusted && btn.val === val", html)
         self.assertNotIn("btn.dataset.bytes === this.value", html)