What exactly are slow in dev mode?

Are we talking about app created via create-react-app?

Думал найду issue с вебпаком

yes create-react-app. input typing very slowly in dev mode

Do You leverage some 3rd party library for type handling, or do You implement it by hand?

I handle by hand

no 3rd liblary

"version": "0.1.0",
 "private": true,
 "engines": {
   "npm": "5.7.1",
   "node": "12.18.2"
 },
 "dependencies": {
   "@babel/compat-data": "^7.11.0",
   "@babel/plugin-proposal-numeric-separator": "^7.10.4",
   "@coreui/react": "^2.5.8",
   "@testing-library/jest-dom": "^4.2.4",
   "@testing-library/react": "^9.3.2",
   "@testing-library/user-event": "^7.1.2",
   "@welldone-software/why-did-you-render": "^4.2.7",
   "antd": "^4.5.2",
   "axios": "^0.19.2",
   "base-64": "^0.1.0",
   "core-js": "^3.6.5",
   "formik": "^2.1.5",
   "i18next": "^19.6.3",
   "i18next-browser-languagedetector": "^4.3.1",
   "i18next-http-backend": "^1.0.17",
   "i18next-xhr-backend": "^3.2.2",
   "jquery": "^3.5.1",
   "lodash": "^4.17.19",
   "momentjs": "^2.0.0",
   "paper-css": "^0.4.1",
   "prop-types": "^15.7.2",
   "qrcode.react": "^1.0.0",
   "react": "^16.13.1",
   "react-app-polyfill": "^1.0.6",
   "react-datepicker": "^2.16.0",
   "react-dom": "^16.13.1",
   "react-error-boundary": "^1.2.5",
   "react-hot-loader": "^4.12.21",
   "react-i18next": "^11.7.0",
   "react-icons": "^3.10.0",
   "react-joyride": "^2.2.1",
   "react-loading-skeleton": "^2.1.1",
   "react-number-format": "^4.4.1",
   "react-pace-progress": "^2.0.0",
   "react-redux": "^7.2.1",
   "react-responsive": "^8.1.0",
   "react-router-dom": "^5.2.0",
   "react-scripts": "^3.4.1",
   "react-select": "^3.1.0",
   "react-spinners": "^0.8.3",
   "react-tabs": "^3.1.1",
   "react-to-print": "^2.9.0",
   "react-toastify": "^6.0.8",
   "redux": "^4.0.5",
   "redux-thunk": "^2.3.0",
   "reselect": "^4.0.0",
   "source-map-explorer": "^2.4.2",
   "styled-components": "^5.1.1",
   "uuid": "^8.3.0",
   "xlsx": "^0.16.5",
   "yarn": "^1.22.4"
 },
 "scripts": {
   "start": "react-scripts start",
   "build": "react-scripts build",
   "test": "react-scripts test",
   "eject": "react-scripts eject",
   "analyze": "source-map-explorer 'build/static/js/*.js'"
 },
 "eslintConfig": {
   "extends": "react-app"
 },
 "browserslist": {
   "production": [
     ">0.2%",
     "not dead",
     "not op_mini all"
   ],
   "development": [
     "last 1 chrome version",
     "last 1 firefox version",
     "last 1 safari version"
   ]
 },
 "devDependencies": {
   "babel-preset-react-app": "^9.1.2",
   "prettier": "2.0.5",
   "react-addons-perf": "^15.4.2"
 }
}
#package.json

I see react-addons-perf in Your package.json file.

Do You measure performance with it?

yes

Maybe this lib report slow-down because in development mode there are unminified version of react that are slower than minified version.

Development vs. Production Builds
If you’re benchmarking or seeing performance problems in your React apps, make sure you’re testing with the minified production build. The development build includes extra warnings that are helpful when building your apps, but it is slower due to the extra bookkeeping it does.

However, the perf tools described on this page only work when using the development build of React. Therefore, the profiler only serves to indicate the relatively expensive parts of your app.

↑ Quote from that page.

yee may be thanks to your opinion

Is this sufficient for You?

of course

Glad to help 🙂

👍