Add basic performance testing via Cypress (#8586)

cypress/plugins/performance.ts

@@ -0,0 +1,47 @@
+/*
+Copyright 2022 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.
+*/
+
+/// <reference types="cypress" />
+
+import * as path from "path";
+import * as fse from "fs-extra";
+
+import PluginEvents = Cypress.PluginEvents;
+import PluginConfigOptions = Cypress.PluginConfigOptions;
+
+// This holds all the performance measurements throughout the run
+let bufferedMeasurements: PerformanceEntry[] = [];
+
+function addMeasurements(measurements: PerformanceEntry[]): void {
+    bufferedMeasurements = bufferedMeasurements.concat(measurements);
+    return null;
+}
+
+async function writeMeasurementsFile() {
+    try {
+        const measurementsPath = path.join("cypress", "performance", "measurements.json");
+        await fse.outputJSON(measurementsPath, bufferedMeasurements, {
+            spaces: 4,
+        });
+    } finally {
+        bufferedMeasurements = [];
+    }
+}
+
+export function performance(on: PluginEvents, config: PluginConfigOptions) {
+    on("task", { addMeasurements });
+    on("after:run", writeMeasurementsFile);
+}