diff --git a/Lib/email/feedparser.py b/Lib/email/feedparser.py
--- a/Lib/email/feedparser.py
+++ b/Lib/email/feedparser.py
@@ -23,6 +23,7 @@
 
 import re
 
+from collections import deque
 from email import errors
 from email import message
 from email._policybase import compat32
@@ -52,8 +53,8 @@
     def __init__(self):
         # The last partial line pushed into this object.
         self._partial = ''
-        # The list of full, pushed lines, in reverse order
-        self._lines = []
+        # A deque of full, pushed lines, in reverse order
+        self._lines = deque()
         # The stack of false-EOF checking predicates.
         self._eofstack = []
         # A flag indicating whether the file has been closed or not.
@@ -82,7 +83,7 @@
         # RFC 2046, section 5.1.2 requires us to recognize outer level
         # boundaries at any level of inner nesting.  Do this, but be sure it's
         # in the order of most to least nested.
-        for ateof in self._eofstack[::-1]:
+        for ateof in reversed(self._eofstack):
             if ateof(line):
                 # We're at the false EOF.  But push the last line back first.
                 self._lines.append(line)
@@ -110,7 +111,7 @@
 
     def pushlines(self, lines):
         # Reverse and insert at the front of the lines.
-        self._lines[:0] = lines[::-1]
+        self._lines.extendleft(lines)
 
     def __iter__(self):
         return self