Really poor performance with Firefox

[ekaradon]

Description

I really like this tool and I use it for all my current projects. I have however a major trouble.
Indeed, when using Firefox with the debugger opened, it just become really slow and it may even freeze as soon as the project fatten.
For example, here with the default project, not even changed:

On this test, I have lost the control of my browser for more than 2 seconds!

Expected behavior

Using Chromium or Firefox should not have a major impact on the performance.

Actual behavior

Using Firefox debugging tools is impossible with the react-app-cli solution.
With my previous tools and the same level of features, it was totally usable.

Environment

➜  demo npm ls react-scripts
demo@0.1.0 /home/ekaradon/projects/comwatt/demo
└── react-scripts@0.6.1 
➜  demo node -v
v6.8.1
➜  demo npm -v
3.10.8
➜  demo uname -a
Linux das_computer 4.7.6-1-ARCH #1 SMP PREEMPT Fri Sep 30 19:28:42 CEST 2016 x86_64 GNU/Linux
➜  demo firefox -v
Mozilla Firefox 49.0.1
➜  demo cat /proc/cpuinfo 
processor       : 0
vendor_id       : GenuineIntel
cpu family      : 6
model           : 94
model name      : Intel(R) Core(TM) i7-6700HQ CPU @ 2.60GHz
➜  demo cat /proc/meminfo 
MemTotal:       16330532 kB
MemFree:         1327896 kB
MemAvailable:    8083516 kB
Buffers:          275356 kB
Cached:          7654904 kB

Reproducible Demo

It is fully reproducible with the standard app created by create-react-app tool.
You just have then to start the application with:
➜ demo npm start
And then, inside Firefox, open the debugging tools (F12) and refresh the page (F5) and watch how much times it get you to have the page displayed.

Cheers,

[gaearon]

Likely due to debug: 'eval'. Since people prefer sourcemaps anyway I guess we’ll switch.
Can you try making this change inside node_modules/react-scripts/config/webpack.config.dev.js, restart the server, and check if it fixes the issue?

[ekaradon]

Hell yeah! It works perfectly now. Thanks for your reactivity!

See here with the default project:

And here with a big project (which was loading for more than one minute before this change):

Is it possible to integrate this change as default in your release?

[gaearon]

Could you send a PR?

[ekaradon]

Hmmm. Ok. Not sure if there is anything more to change? But I will do it.

[gaearon]

Probably nothing else.

[ekaradon]

@gaearon: Done. Thank you again for the help!

[gaearon]

Fixed via #924, will be out in 0.8.0.