This document highlights important aspects of the "Timing" tab.

Be Cautious And Sceptical

There are two issues with the displayed numbers:

1. Accurately timing something in the browser is almost a fool's errand. One moment it takes 1ms and the next it takes 10ms, and you’ll never know why. Noisy.

   So, don't ever base your decisions on one set of timings. Run the same event at least a few of times.

   In the future, we'd like to add a 'Run It Again' button, which you can click a few times to see if you get stable numbers. Perhaps you'll beat us to it, and create a PR for this feature?

2. Don't freak out about any apparent slowness, yet.

   After all, you're running a dev build, right, not the production build? And I'm guessing you're also running a dev build of React?

   And using re-frame-trace will slow things down too, what with all that creating and analysing of trace.

   So, run the production version of your app first, before deciding you have a performance problem. Something what takes 100ms in dev might take 20ms in prod.

   The Timing Tab is not really about absolute numbers so much as the relative time taken to do the different "parts" of an Epoch. Is one View very slow for some reason, compared to others? And, even then, remember point 1 (above).

Know Your Epoch Timeline

The Timing Tab is easier to understand once you have internalised the following graphic which shows how, operationally, the six dominoes play out, over time, within the browser.

Other Tips

You should probably have React DevTools installed because it is useful. But, it can also add drag and noise to timing results, so disable it when trying to get more accurate timing figures.

Here is (React 16) advice on debugging React performance with Chrome Devtools

The re-frame.core/debug middleware is relatively slow, and runs interleaved with your application's events being processed. re-frame-trace gives you the same information in the app-db panel, but saves the calculations until after your application has finished running, so you don't get the performance cost included in your timing.