next.js/bench/fixtures/basic/pages/css.js

import React, { Component } from 'react'
import { StyleSheet, css } from 'next/css'

export default class CrazyCSS extends Component {
  spans () {
    const out = []
    for (let i = 0; i < 1000; i++) {
      out.push(<span key={i} class={css(styles[`padding-${i}`])}>This is ${i}</span>)
    }
    return out
  }

  render () {
    return <div>{this.spans()}</div>
  }
}

const spanStyles = {}
for (let i = 0; i < 1000; i++) {
  spanStyles[`padding-${i}`] = { padding: i }
}

const styles = StyleSheet.create(spanStyles)