Measure performance of web application from mobile

Question

Want to measure the performance of a web application for mobile devices (designed in Microsoft PowerApps) from my android and iOS devices. Basically, I am more interested toward

Answer 1

To measure key web vitals

There is a great library by Google to monitor web vitals. It lets you monitor in real time and you can even pipe the info to Google Analytics or your own server.

This monitors

Cumulative Layout Shift (CLS)
First Input Delay (FID)
Largest Contentful Paint (LCP)
First Contentful Paint (FCP)
Time to First Byte (TTFB)

To measure other metrics

As for other metrics they can (nearly) all be derived from window.performance.

This gives you all the information you need to calculate load times etc. (taken from this answer):

connectEnd                 Time when server connection is finished.
connectStart               Time just before server connection begins.
domComplete                Time just before document readiness completes.
domContentLoadedEventEnd   Time after DOMContentLoaded event completes.
domContentLoadedEventStart Time just before DOMContentLoaded starts.
domInteractive             Time just before readiness set to interactive.
domLoading                 Time just before readiness set to loading.
domainLookupEnd            Time after domain name lookup.
domainLookupStart          Time just before domain name lookup.
fetchStart                 Time when the resource starts being fetched.
loadEventEnd               Time when the load event is complete.
loadEventStart             Time just before the load event is fired.
navigationStart            Time after the previous document begins unload.
redirectCount              Number of redirects since the last non-redirect.
redirectEnd                Time after last redirect response ends.
redirectStart              Time of fetch that initiated a redirect.
requestStart               Time just before a server request.
responseEnd                Time after the end of a response or connection.
responseStart              Time just before the start of a response.
timing                     Reference to a performance timing object.
navigation                 Reference to performance navigation object.
performance                Reference to performance object for a window.
type                       Type of the last non-redirect navigation event.
unloadEventEnd             Time after the previous document is unloaded.
unloadEventStart           Time just before the unload event is fired.

The two difficult ones to calculate

The only thing you can't calculate with the above two items is Total Blocking Time (TBT) and Time To Interactive (TTI).

For that you would need to use PerformanceObserver and find all long tasks (greater than 50ms) between First Contentful Paint and Time To Interactive.

Time to Interactive is equally complicated and you would have to do some research on that (maybe start here) as it is quite hard to implement and I don't bother tracking it on live stats.

Or you could just adapt this polyfill for TTI