Performance

var now = await performance.Now();          // e.g. 5321.4 ms since navigation
var origin = await performance.TimeOrigin(); // Unix epoch ms

var startedAt = DateTimeOffset.FromUnixTimeMilliseconds((long)origin);

await performance.Mark("checkout:start");

// ... do the work you want to time ...

await performance.Mark("checkout:end");

await performance.Measure("checkout", "checkout:start", "checkout:end");

var entries = await performance.GetEntries("checkout", "measure");
var duration = entries[^1].GetProperty("duration").GetDouble();

var all = await performance.GetEntries();
var resources = await performance.GetEntries(type: "resource");
var myMarks = await performance.GetEntries("checkout:start", "mark");

PerformanceNavigationTiming[] nav = await performance.GetNavigationEntries();

if (nav.Length > 0)
{
    var page = nav[0];
    var ttfb = page.ResponseStart - page.RequestStart;
    var dom = page.DomContentLoadedEventEnd;
    var loaded = page.LoadEventEnd;
}

PerformanceResourceTiming[] resources = await performance.GetResourceEntries();

var slowest = resources.OrderByDescending(r => r.Duration).First();
// slowest.InitiatorType, slowest.NextHopProtocol, slowest.TransferSize

// or narrow to one URL:
var one = await performance.GetResourceEntries("https://example.com/app.js");

WebVitals? vitals = await performance.GetWebVitals(); // null during prerender/SSR

// vitals.Lcp  - milliseconds, good below 2,500
// vitals.Cls  - unitless, good below 0.1
// vitals.Inp  - milliseconds, good below 200; null until the user interacts
// a metric the engine does not implement is null, which is not a zero score

// typed one-shot reads
LargestContentfulPaint[] lcp = await performance.GetLargestContentfulPaints();
PerformanceEventTiming[] slow = await performance.GetEventTimings();
PerformanceEntry[] paints = await performance.GetTypedEntries<PerformanceEntry>(PerformanceEntryTypes.Paint);

// the kinds that are only ever observed
var subscription = await performance.SubscribeObserver<PerformanceLongTaskTiming>(
    [PerformanceEntryTypes.LongTask],
    tasks =>
    {
        foreach (var task in tasks)
        {
            Console.WriteLine($"{task.Duration:F0} ms blocked by {task.Name}");
        }
    });

await subscription.DisposeAsync();

var subscription = await performance.SubscribeObserver(
    ["mark", "measure"],
    entries =>
    {
        foreach (var entry in entries)
        {
            var name = entry.GetProperty("name").GetString();
            var duration = entry.GetProperty("duration").GetDouble();
        }
    },
    buffered: true);

// later, when no longer needed:
await subscription.DisposeAsync();

var memory = await performance.GetMemory(); // null during prerender/SSR

// memory.UsedJsHeapSize, memory.TotalJsHeapSize, memory.JsHeapSizeLimit
// all fields are null when the browser does not expose performance.memory

await performance.ClearMarks();            // all marks
await performance.ClearMarks("checkout:start"); // a single mark
await performance.ClearMeasures();
await performance.ClearResourceTimings();