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They harp a lot on how fast it starts up - which sounds great - but I would like to have some solid numbers on what that startup time actually is on actual devices! I can't seem to find anything perusing around the GH page or the site



We have done some internal tests of Hermes on a RN experience within Microsoft Office on Android.

Currently we use V8 with bytecode caching on Android, since it provided better startup performance than the JSC engine that normally ships within RN. So the baseline is likely already faster than stock RN.

V8 runtime Memory Impact: 30MB Hermes runtime memory impact: 21.5MB

V8 time to interaction: 1.4s Hermes time to interaction: 1.1s

We have done similar experiments on a full react-native-windows application, replacing the Chakra JS engine (also already with bytecode so faster than stock RN) with Hermes and had app boot time on a low end device go from 3.8s to 3.1s.


It's a pity the Proxy is not supported mobx is really much nicer approach to state management and usually, apps using mobx are much faster - simply because implementing shouldComponentUpdate properly is hard


You could still use MobX 4 though. It's equally well maintained.


Very nice numbers, although worth mentioning hermes only appears to support es5, so there's way less stuff to load.


Hermes compiles JS ahead of time to bytecode. The VM takes bytecode in as input, not source code. The ES5 vs. ES6 distinction doesn't matter as much as in other engines where the runtime takes source code as input, and as a result has to pay the more expensive ES6 parsing cost at runtime.



Since it's optimized for reducing memory consumption, you'd expect it to make boot time worse?


Impressive numbers. Did you try to use startup snapshot in V8 to improve TTI? https://v8.dev/blog/custom-startup-snapshots


From your numbers:

1. runtime memory reduction: ~25%

2. Boot time reduction: 20%

3. Time to interaction reduction: 20%

For apps with significant boot times, and runtime memory consumption, this is valuable.


For apps with significant boot times, and runtime memory consumption, this is valuable.

This assumes that the reductions are purely proportional to the total and not (at least partially) fixed.


Here are some details on Hermes perf with sample code:

https://mobile.twitter.com/htormey/status/114935339023051980...

Here is a video comparing start times for a Hermes/non Hermes RN app:

https://mobile.twitter.com/nparashuram/status/11493502142154...

I’m currently at Chain React in Portland and they just gave a talk on Hermes. I tweeted a few screenshots that show other perf numbers/technical details for those who are interested.


Thanks for the links!




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