re-frame-10x/docs/HyperlinkedInformation/UnderstandingTiming.md

This document explains useful things about the "Timing" tab.

Be Cautious And Sceptical

There are two issues with the 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 decision 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 that takes 100ms in dev might take 20ms in prod.

   This 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 ridiculously slow for some reason, compared to others? And, even then, remember point 1 (above).

Know Your Epoch Timeline

You'll understand the contents of the Timings tab better if you understand how an event is processed over time within the browser. The following infographic will help: