diff --git a/src/ContentMessages.tsx b/src/ContentMessages.tsx
index 14a0c1ed51..40f8e307a5 100644
--- a/src/ContentMessages.tsx
+++ b/src/ContentMessages.tsx
@@ -39,6 +39,8 @@ import {
 import { IUpload } from "./models/IUpload";
 import { IAbortablePromise, IImageInfo } from "matrix-js-sdk/src/@types/partials";
 import { BlurhashEncoder } from "./BlurhashEncoder";
+import SettingsStore from "./settings/SettingsStore";
+import { decorateStartSendingTime, sendRoundTripMetric } from "./sendTimePerformanceMetrics";
 
 const MAX_WIDTH = 800;
 const MAX_HEIGHT = 600;
@@ -539,6 +541,10 @@ export default class ContentMessages {
             msgtype: "", // set later
         };
 
+        if (SettingsStore.getValue("Performance.addSendMessageTimingMetadata")) {
+            decorateStartSendingTime(content);
+        }
+
         // if we have a mime type for the file, add it to the message metadata
         if (file.type) {
             content.info.mimetype = file.type;
@@ -614,6 +620,11 @@ export default class ContentMessages {
         }).then(function() {
             if (upload.canceled) throw new UploadCanceledError();
             const prom = matrixClient.sendMessage(roomId, content);
+            if (SettingsStore.getValue("Performance.addSendMessageTimingMetadata")) {
+                prom.then(resp => {
+                    sendRoundTripMetric(matrixClient, roomId, resp.event_id);
+                });
+            }
             CountlyAnalytics.instance.trackSendMessage(startTime, prom, roomId, false, false, content);
             return prom;
         }, function(err) {
diff --git a/src/components/views/rooms/SendMessageComposer.tsx b/src/components/views/rooms/SendMessageComposer.tsx
index aca397b6b2..bb5d537895 100644
--- a/src/components/views/rooms/SendMessageComposer.tsx
+++ b/src/components/views/rooms/SendMessageComposer.tsx
@@ -54,6 +54,7 @@ import { Room } from 'matrix-js-sdk/src/models/room';
 import ErrorDialog from "../dialogs/ErrorDialog";
 import QuestionDialog from "../dialogs/QuestionDialog";
 import { ActionPayload } from "../../../dispatcher/payloads";
+import { decorateStartSendingTime, sendRoundTripMetric } from "../../../sendTimePerformanceMetrics";
 
 function addReplyToMessageContent(
     content: IContent,
@@ -418,6 +419,10 @@ export default class SendMessageComposer extends React.Component<IProps> {
             // don't bother sending an empty message
             if (!content.body.trim()) return;
 
+            if (SettingsStore.getValue("Performance.addSendMessageTimingMetadata")) {
+                decorateStartSendingTime(content);
+            }
+
             const prom = this.context.sendMessage(roomId, content);
             if (replyToEvent) {
                 // Clear reply_to_event as we put the message into the queue
@@ -433,6 +438,11 @@ export default class SendMessageComposer extends React.Component<IProps> {
                     dis.dispatch({ action: `effects.${effect.command}` });
                 }
             });
+            if (SettingsStore.getValue("Performance.addSendMessageTimingMetadata")) {
+                prom.then(resp => {
+                    sendRoundTripMetric(this.context, roomId, resp.event_id);
+                });
+            }
             CountlyAnalytics.instance.trackSendMessage(startTime, prom, roomId, false, !!replyToEvent, content);
         }
 
diff --git a/src/sendTimePerformanceMetrics.ts b/src/sendTimePerformanceMetrics.ts
new file mode 100644
index 0000000000..ef461db939
--- /dev/null
+++ b/src/sendTimePerformanceMetrics.ts
@@ -0,0 +1,48 @@
+/*
+Copyright 2021 The Matrix.org Foundation C.I.C.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+import { MatrixClient } from "matrix-js-sdk";
+
+/**
+ * Decorates the given event content object with the "send start time". The
+ * object will be modified in-place.
+ * @param {object} content The event content.
+ */
+export function decorateStartSendingTime(content: object) {
+    content['io.element.performance_metrics'] = {
+        sendStartTs: Date.now(),
+    };
+}
+
+/**
+ * Called when an event decorated with `decorateStartSendingTime()` has been sent
+ * by the server (the client now knows the event ID).
+ * @param {MatrixClient} client The client to send as.
+ * @param {string} inRoomId The room ID where the original event was sent.
+ * @param {string} forEventId The event ID for the decorated event.
+ */
+export function sendRoundTripMetric(client: MatrixClient, inRoomId: string, forEventId: string) {
+    // noinspection JSIgnoredPromiseFromCall
+    client.sendEvent(inRoomId, 'io.element.performance_metric', {
+        // XXX: We stick all of this into `m.relates_to` so it doesn't end up encrypted.
+        "m.relates_to": {
+            rel_type: "io.element.metric",
+            event_id: forEventId,
+            responseTs: Date.now(),
+            kind: 'send_time',
+        } as any, // override types because we're actually allowed to add extra metadata to relates_to
+    });
+}
diff --git a/src/settings/Settings.tsx b/src/settings/Settings.tsx
index 40f57a0a1c..6dbefd4b8e 100644
--- a/src/settings/Settings.tsx
+++ b/src/settings/Settings.tsx
@@ -759,6 +759,10 @@ export const SETTINGS: {[setting: string]: ISetting} = {
         default: true,
         controller: new ReducedMotionController(),
     },
+    "Performance.addSendMessageTimingMetadata": {
+        supportedLevels: [SettingLevel.CONFIG],
+        default: false,
+    },
     "Widgets.pinned": { // deprecated
         supportedLevels: LEVELS_ROOM_OR_ACCOUNT,
         default: {},