diff --git a/changelog.d/16429.misc b/changelog.d/16429.misc
new file mode 100644
index 000000000..4241e143b
--- /dev/null
+++ b/changelog.d/16429.misc
@@ -0,0 +1 @@
+Reduce the size of each replication command instance.
diff --git a/synapse/replication/tcp/commands.py b/synapse/replication/tcp/commands.py
index e616b5e1c..1b92302fd 100644
--- a/synapse/replication/tcp/commands.py
+++ b/synapse/replication/tcp/commands.py
@@ -18,7 +18,7 @@ allowed to be sent by which side.
 """
 import abc
 import logging
-from typing import Optional, Tuple, Type, TypeVar
+from typing import List, Optional, Tuple, Type, TypeVar
 
 from synapse.replication.tcp.streams._base import StreamRow
 from synapse.util import json_decoder, json_encoder
@@ -74,6 +74,8 @@ SC = TypeVar("SC", bound="_SimpleCommand")
 class _SimpleCommand(Command):
     """An implementation of Command whose argument is just a 'data' string."""
 
+    __slots__ = ["data"]
+
     def __init__(self, data: str):
         self.data = data
 
@@ -122,6 +124,8 @@ class RdataCommand(Command):
         RDATA presence master 59 ["@baz:example.com", "online", ...]
     """
 
+    __slots__ = ["stream_name", "instance_name", "token", "row"]
+
     NAME = "RDATA"
 
     def __init__(
@@ -179,6 +183,8 @@ class PositionCommand(Command):
     of the stream.
     """
 
+    __slots__ = ["stream_name", "instance_name", "prev_token", "new_token"]
+
     NAME = "POSITION"
 
     def __init__(
@@ -235,6 +241,8 @@ class ReplicateCommand(Command):
         REPLICATE
     """
 
+    __slots__: List[str] = []
+
     NAME = "REPLICATE"
 
     def __init__(self) -> None:
@@ -264,6 +272,8 @@ class UserSyncCommand(Command):
     Where <state> is either "start" or "end"
     """
 
+    __slots__ = ["instance_id", "user_id", "device_id", "is_syncing", "last_sync_ms"]
+
     NAME = "USER_SYNC"
 
     def __init__(
@@ -316,6 +326,8 @@ class ClearUserSyncsCommand(Command):
         CLEAR_USER_SYNC <instance_id>
     """
 
+    __slots__ = ["instance_id"]
+
     NAME = "CLEAR_USER_SYNC"
 
     def __init__(self, instance_id: str):
@@ -343,6 +355,8 @@ class FederationAckCommand(Command):
         FEDERATION_ACK <instance_name> <token>
     """
 
+    __slots__ = ["instance_name", "token"]
+
     NAME = "FEDERATION_ACK"
 
     def __init__(self, instance_name: str, token: int):
@@ -368,6 +382,15 @@ class UserIpCommand(Command):
         USER_IP <user_id>, <access_token>, <ip>, <device_id>, <last_seen>, <user_agent>
     """
 
+    __slots__ = [
+        "user_id",
+        "access_token",
+        "ip",
+        "user_agent",
+        "device_id",
+        "last_seen",
+    ]
+
     NAME = "USER_IP"
 
     def __init__(
@@ -441,6 +464,8 @@ class LockReleasedCommand(Command):
         LOCK_RELEASED ["<instance_name>", "<lock_name>", "<lock_key>"]
     """
 
+    __slots__ = ["instance_name", "lock_name", "lock_key"]
+
     NAME = "LOCK_RELEASED"
 
     def __init__(