diff --git a/HISTORY.md b/HISTORY.md
index 49049706f5..5f1f27690f 100644
--- a/HISTORY.md
+++ b/HISTORY.md
@@ -1,6 +1,13 @@
 Release History
 ===============
 
+3.6.4 (2024-05-16)
+------------------
+
+**Changed**
+- Avoid parsing X509 peer certificate in the certificate revocation check process over and over again.
+- Avoid iterating over header items redundantly or needlessly.
+
 3.6.3 (2024-05-06)
 ------------------
 
diff --git a/pyproject.toml b/pyproject.toml
index b2556726ab..67bc9e5ffd 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -8,7 +8,7 @@ description = "Niquests is a simple, yet elegant, HTTP library. It is a drop-in
 readme = "README.md"
 license-files = { paths = ["LICENSE"] }
 license = "Apache-2.0"
-keywords = ["requests", "http/2", "http/3", "QUIC", "http", "https", "http client", "http/1.1", "ocsp", "revocation", "tls", "multiplexed", "dns-over-quic", "doq", "dns-over-tls", "dot", "dns-over-https", "doh", "dnssec"]
+keywords = ["requests", "http2", "http3", "QUIC", "http", "https", "http client", "http/1.1", "ocsp", "revocation", "tls", "multiplexed", "dns-over-quic", "doq", "dns-over-tls", "dot", "dns-over-https", "doh", "dnssec"]
 authors = [
   {name = "Kenneth Reitz", email = "me@kennethreitz.org"}
 ]
diff --git a/src/niquests/__version__.py b/src/niquests/__version__.py
index b73b75b459..12078d1119 100644
--- a/src/niquests/__version__.py
+++ b/src/niquests/__version__.py
@@ -9,9 +9,9 @@
 __url__: str = "https://niquests.readthedocs.io"
 
 __version__: str
-__version__ = "3.6.3"
+__version__ = "3.6.4"
 
-__build__: int = 0x030603
+__build__: int = 0x030604
 __author__: str = "Kenneth Reitz"
 __author_email__: str = "me@kennethreitz.org"
 __license__: str = "Apache-2.0"
diff --git a/src/niquests/extensions/_async_ocsp.py b/src/niquests/extensions/_async_ocsp.py
index 6c47125490..810f588c29 100644
--- a/src/niquests/extensions/_async_ocsp.py
+++ b/src/niquests/extensions/_async_ocsp.py
@@ -53,7 +53,11 @@
     async_recv_tls_and_decrypt,
     async_send_tls,
 )
-from ._ocsp import _str_fingerprint_of, readable_revocation_reason
+from ._ocsp import (
+    _str_fingerprint_of,
+    readable_revocation_reason,
+    _parse_x509_der_cached,
+)
 
 
 async def _ask_nicely_for_issuer(
@@ -320,7 +324,7 @@ async def verify(
     if not endpoints:
         return
 
-    peer_certificate = Certificate(conn_info.certificate_der)
+    peer_certificate = _parse_x509_der_cached(conn_info.certificate_der)
 
     async with _SharedRevocationStatusCache.lock(peer_certificate):
         # this feature, by default, is reserved for a reasonable usage.
diff --git a/src/niquests/extensions/_ocsp.py b/src/niquests/extensions/_ocsp.py
index e78ff48633..a59a76b49d 100644
--- a/src/niquests/extensions/_ocsp.py
+++ b/src/niquests/extensions/_ocsp.py
@@ -9,6 +9,7 @@
 from hashlib import sha256
 from random import randint
 from statistics import mean
+from functools import lru_cache
 
 from qh3._hazmat import (
     OCSPRequest,
@@ -52,6 +53,11 @@
 )
 
 
+@lru_cache(maxsize=64)
+def _parse_x509_der_cached(der: bytes) -> Certificate:
+    return Certificate(der)
+
+
 def _str_fingerprint_of(certificate: Certificate) -> str:
     return ":".join(
         [format(i, "02x") for i in sha256(certificate.public_bytes()).digest()]
@@ -327,7 +333,7 @@ def verify(
         if _SharedRevocationStatusCache.hold:
             return
 
-    peer_certificate = Certificate(conn_info.certificate_der)
+    peer_certificate = _parse_x509_der_cached(conn_info.certificate_der)
     cached_response = _SharedRevocationStatusCache.check(peer_certificate)
 
     if cached_response is not None:
diff --git a/src/niquests/sessions.py b/src/niquests/sessions.py
index 3263a9d8df..7beb5d58e5 100644
--- a/src/niquests/sessions.py
+++ b/src/niquests/sessions.py
@@ -128,17 +128,26 @@ def merge_setting(
         return session_setting
 
     # Bypass if not a dictionary (e.g. verify)
-    if not (
+    if isinstance(session_setting, bool) or not (
         isinstance(session_setting, Mapping) and isinstance(request_setting, Mapping)
     ):
         return request_setting
 
-    merged_setting = dict_class(to_key_val_list(session_setting))
+    if hasattr(session_setting, "copy"):
+        merged_setting = (
+            session_setting.copy()
+            if session_setting.__class__ is dict_class
+            else dict_class(session_setting.copy())
+        )
+    else:
+        merged_setting = dict_class(to_key_val_list(session_setting))
+
     merged_setting.update(to_key_val_list(request_setting))
 
     # Remove keys that are set to None. Extract keys first to avoid altering
     # the dictionary during iteration.
-    none_keys = [k for (k, v) in merged_setting.items() if v is None]
+    none_keys = [k for k in merged_setting if merged_setting[k] is None]
+
     for key in none_keys:
         del merged_setting[key]
 
diff --git a/src/niquests/structures.py b/src/niquests/structures.py
index 949149bd5b..3c48f207e1 100644
--- a/src/niquests/structures.py
+++ b/src/niquests/structures.py
@@ -126,6 +126,10 @@ def lower_items(self) -> typing.Iterator[tuple[bytes | str, bytes | str]]:
         """Like iteritems(), but with all lowercase keys."""
         return ((lowerkey, keyval[1]) for (lowerkey, keyval) in self._store.items())
 
+    def items(self):
+        for k in self._store:
+            yield self._store[k]
+
     def __eq__(self, other) -> bool:
         if isinstance(other, Mapping):
             other = CaseInsensitiveDict(other)
@@ -136,7 +140,7 @@ def __eq__(self, other) -> bool:
 
     # Copy is required
     def copy(self) -> CaseInsensitiveDict:
-        return CaseInsensitiveDict(self._store.values())
+        return CaseInsensitiveDict(self)
 
     def __repr__(self) -> str:
         return str(dict(self.items()))