From 5cb7135a9f3c8efa21972a72b7acfcc4e6343cca Mon Sep 17 00:00:00 2001
From: Quentin Gliech <quenting@element.io>
Date: Wed, 4 Mar 2026 11:07:25 +0100
Subject: [PATCH 1/2] Avoid re-computing the event ID when cloning events.

---
 synapse/events/utils.py | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/synapse/events/utils.py b/synapse/events/utils.py
index b79a68f589a..1bf4d632c00 100644
--- a/synapse/events/utils.py
+++ b/synapse/events/utils.py
@@ -48,7 +48,7 @@
 from synapse.logging.opentracing import SynapseTags, set_tag, trace
 from synapse.types import JsonDict, Requester
 
-from . import EventBase, StrippedStateEvent, make_event_from_dict
+from . import EventBase, FrozenEventV2, StrippedStateEvent, make_event_from_dict
 
 if TYPE_CHECKING:
     from synapse.handlers.relations import BundledAggregations
@@ -109,6 +109,14 @@ def clone_event(event: EventBase) -> EventBase:
         event.get_dict(), event.room_version, event.internal_metadata.get_dict()
     )
 
+    # Starting FrozenEventV2, the event ID is an (expensive) hash of the event. This is
+    # lazily computed when we get the FrozenEventV2.event_id property, then cached in
+    # _event_id field. Later FrozenEvent formats all inherit from FrozenEventV2, so we
+    # can use the same logic here.
+    if isinstance(event, FrozenEventV2) and isinstance(new_event, FrozenEventV2):
+        # If we already pre-computed the event ID, use it.
+        new_event._event_id = event._event_id
+
     # Copy the bits of `internal_metadata` that aren't returned by `get_dict`.
     new_event.internal_metadata.stream_ordering = (
         event.internal_metadata.stream_ordering

From 57508885707b23e64bc2c04ccf5c1bbe9dc591f4 Mon Sep 17 00:00:00 2001
From: Quentin Gliech <quenting@element.io>
Date: Wed, 4 Mar 2026 11:36:48 +0100
Subject: [PATCH 2/2] Newsfile

---
 changelog.d/19527.misc | 1 +
 1 file changed, 1 insertion(+)
 create mode 100644 changelog.d/19527.misc

diff --git a/changelog.d/19527.misc b/changelog.d/19527.misc
new file mode 100644
index 00000000000..c349af286b1
--- /dev/null
+++ b/changelog.d/19527.misc
@@ -0,0 +1 @@
+Avoid re-computing the event ID when cloning events.