diff --git a/Lib/email/header.py b/Lib/email/header.py
index ea1f84ea964ca60..9937b6d4f3a0cf1 100644
--- a/Lib/email/header.py
+++ b/Lib/email/header.py
@@ -546,24 +546,31 @@ class _Accumulator(list):
 
     def __init__(self, initial_size=0):
         self._initial_size = initial_size
+        # The value of len(): _initial_size plus the length of all the
+        # parts, kept up to date by push(), pop(), pop_from() and reset().
+        # Summing the parts on every call makes folding quadratic.
+        self._len = initial_size
         super().__init__()
 
     def push(self, fws, string):
         self.append((fws, string))
+        self._len += len(fws) + len(string)
 
     def pop_from(self, i=0):
         popped = self[i:]
         self[i:] = []
+        self._len -= sum(len(fws) + len(part) for fws, part in popped)
         return popped
 
     def pop(self):
         if self.part_count()==0:
             return ('', '')
-        return super().pop()
+        fws, part = super().pop()
+        self._len -= len(fws) + len(part)
+        return fws, part
 
     def __len__(self):
-        return sum((len(fws)+len(part) for fws, part in self),
-                   self._initial_size)
+        return self._len
 
     def __str__(self):
         return EMPTYSTRING.join((EMPTYSTRING.join((fws, part))
@@ -574,6 +581,7 @@ def reset(self, startval=None):
             startval = []
         self[:] = startval
         self._initial_size = 0
+        self._len = sum(len(fws) + len(part) for fws, part in self)
 
     def is_onlyws(self):
         return self._initial_size==0 and (not self or str(self).isspace())
diff --git a/Lib/test/test_email/test_email.py b/Lib/test/test_email/test_email.py
index 2a5794a5b66fc5b..a5954a269fa6409 100644
--- a/Lib/test/test_email/test_email.py
+++ b/Lib/test/test_email/test_email.py
@@ -1290,6 +1290,35 @@ def test_no_split_long_header(self):
         h = Header('x' * 80)
         eq(h.encode(), 'xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx')
 
+    def test_unfolded_long_header(self):
+        eq = self.ndiffAssertEqual
+        value = ' '.join(f'<id{i}@example.com>' for i in range(2000))
+        h = Header(value, header_name='References')
+        eq(h.encode(maxlinelen=0), value)
+        msg = Message()
+        msg['References'] = value
+        eq(msg.as_string(), f'References: {value}\n\n')
+
+    def test_accumulator_length(self):
+        from email.header import _Accumulator
+        acc = _Accumulator(10)
+        self.assertEqual(len(acc), 10)
+        acc.push(' ', 'abc')
+        acc.push('\t', 'de')
+        acc.push('', 'f')
+        self.assertEqual(len(acc), 10 + 4 + 3 + 1)
+        self.assertEqual(acc.pop(), ('', 'f'))
+        self.assertEqual(len(acc), 10 + 4 + 3)
+        self.assertEqual(acc.pop_from(1), [('\t', 'de')])
+        self.assertEqual(len(acc), 10 + 4)
+        acc.reset([(' ', 'gh'), ('', 'ijk')])
+        self.assertEqual(len(acc), 6)
+        self.assertEqual(acc.part_count(), 2)
+        acc.reset()
+        self.assertEqual(len(acc), 0)
+        self.assertEqual(acc.pop(), ('', ''))
+        self.assertEqual(len(acc), 0)
+
     def test_splitting_multiple_long_lines(self):
         eq = self.ndiffAssertEqual
         hstr = """\
diff --git a/Misc/NEWS.d/next/Library/2026-11-04-00-40-27.gh-issue-161284.o6JvHQ.rst b/Misc/NEWS.d/next/Library/2026-11-04-00-40-27.gh-issue-161284.o6JvHQ.rst
new file mode 100644
index 000000000000000..f0732224da0a6e4
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2026-11-04-00-40-27.gh-issue-161284.o6JvHQ.rst
@@ -0,0 +1,2 @@
+Fix quadratic time complexity in :meth:`email.header.Header.encode` when
+folding long headers.  This also affects :meth:`email.message.Message.as_string`.