diff --git a/benchmarks/TernarySearchTree.mjs b/benchmarks/TernarySearchTree.mjs
new file mode 100644
index 00000000000..413288a6af3
--- /dev/null
+++ b/benchmarks/TernarySearchTree.mjs
@@ -0,0 +1,20 @@
+import { bench, group, run } from 'mitata'
+import { tree } from '../lib/core/tree.js'
+
+const contentLength = Buffer.from('Content-Length')
+const contentLengthUpperCase = Buffer.from('Content-Length'.toUpperCase())
+const contentLengthLowerCase = Buffer.from('Content-Length'.toLowerCase())
+
+group('tree.search', () => {
+  bench('content-length', () => {
+    tree.lookup(contentLengthLowerCase)
+  })
+  bench('CONTENT-LENGTH', () => {
+    tree.lookup(contentLengthUpperCase)
+  })
+  bench('Content-Length', () => {
+    tree.lookup(contentLength)
+  })
+})
+
+await run()
diff --git a/lib/core/tree.js b/lib/core/tree.js
index 366fc7d3207..9b50767c6d3 100644
--- a/lib/core/tree.js
+++ b/lib/core/tree.js
@@ -83,7 +83,10 @@ class TstNode {
     while (node !== null && index < keylength) {
       let code = key[index]
       // A-Z
-      if (code >= 0x41 && code <= 0x5a) {
+      // First check if it is bigger than 0x5a.
+      // Lowercase letters have higher char codes than uppercase ones.
+      // Also we assume that headers will mostly contain lowercase characters.
+      if (code <= 0x5a && code >= 0x41) {
         // Lowercase for uppercase.
         code |= 32
       }
@@ -121,6 +124,7 @@ class TernarySearchTree {
 
   /**
    * @param {Uint8Array} key
+   * @return {any}
    */
   lookup (key) {
     return this.node?.search(key)?.value ?? null
diff --git a/test/node-test/tree.js b/test/node-test/tree.js
index eee3fa85eac..44a7d7960ac 100644
--- a/test/node-test/tree.js
+++ b/test/node-test/tree.js
@@ -13,6 +13,13 @@ describe('Ternary Search Tree', () => {
     assert.throws(() => tst.insert(Buffer.from(''), ''))
   })
 
+  test('looking up not inserted key returns null', () => {
+    assert.throws(() => new TernarySearchTree().insert(Buffer.from(''), ''))
+    const tst = new TernarySearchTree()
+    tst.insert(Buffer.from('a'), 'a')
+    assert.strictEqual(tst.lookup(Buffer.from('non-existant')), null)
+  })
+
   test('duplicate key', () => {
     const tst = new TernarySearchTree()
     const key = Buffer.from('a')