Incorrect frame rate logs on Pavlovia

Online experiments
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8

This works perfectly, thank you very much for the help!
I tweaked it a little bit to change how the values are logged, I’ll post below what I ended up adding to my Instructions routine (which is mostly your code), in case anyone else needs it - the difference here I guess is that the interval for averaging is variable e.g. for as long as the routine is viewed:

Begin Experiment:

window.refreshCount = 0;
window.refreshStart = null;
window.refreshFinish = null;

Each Frame:

    if (window.refreshStart === null) {
        window.refreshStart = window.performance.now();
     } else {
        window.refreshFinish = window.performance.now();
        window.refreshCount++;    
     }

    console.log(window.refreshCount);
    console.log(window.performance.now());

End Routine:

let duration = window.refreshFinish - window.refreshStart;
expInfo['realFrameRate'] = (duration / window.refreshCount);