I am trying to benchmark the performance implications of various iframe setups.
I would like to know how long it takes from the start of the root document until a certain point within an iframe.
One option is to use Date.now() and compare that to the host's performance.timeOrigin of the root page like so:
<body>
<iframe> <!-- cross origin iframe -->
<script>console.log(Date.now())</script>
</iframe>
<script>console.log(performance.timeOrigin)</script>
</body>
The issue is that Date.now() offers inaccurate times and I would rather use a high resolution timer offered by the performance API.
Trying to use performance.now() gives me a relative duration since the start of the current frame, however I am trying to obtain the relative duration since the start of the rootmost document.
So if you imagine something like this:
<body>
<iframe>
<script>console.log(performance.now())</script> // 8ms
</iframe>
<script>console.log(performance.now())</script> // 30ms
</body>
There does not appear to be a way to get a high resolution absolute time using the performance API. Any ideas?