A simple snippet for a time based test.

performanceTest.js

describe('My awesome new way of doing things', () => {
  it('should be performant.' () => {
    const maxRunTimeMs = 500;
    const start = process.hrtime();
    return doSomethingAwesome()
      .then(() => {
        const runtime = process.hrtime(start);
        const runtimeNs = (runtime[0] * 1e9) + runtime[1];
        const runtimeMs = runtimeNs / 1e6;
        expect(runtimeMs).to.be.lessThan(maxRunTimeMs);
      });
  });
});