Slow tests when not mocked with factory method

[cpitt]

🐛 Bug Report

We use Jest with an un-ejected CRA 2.0 configuration and have noticed extremely slow test speeds when component dependencies are not manually mocked with jest.mock using a factory method.

We added manual mocks with factory methods into a single domain of our code and saw drastic speed improvements. Attempts to use __mock__ directories made no impact.

Below are the results for only that test domain
Before (unmocked):

Test Suites: 74 passed, 74 total
Tests:       183 passed, 183 total
Snapshots:   0 total
Time:        105.903s
Ran all test suites matching /Notifications/i.
✨  Done in 107.97s.

After (mocked with factory methods):

Test Suites: 74 passed, 74 total
Tests:       183 passed, 183 total
Snapshots:   0 total
Time:        14.547s
Ran all test suites matching /Notifications/i.
✨  Done in 17.18s.                                                                                                                                                                         

Our current hypothesis is this is due to slow module resolution as tests for components with deeper dependency trees see a more drastic improvement when mocked with a factory method, whereas components with shallower dependencies see only marginal improvements.

To Reproduce

1. Create a component that uses several subcomponents (we're using material-ui, but we've seen similar results without libraries)
2. Run test and record speed
3. Add jest.mock with factory function
   Example:

jest.mock('@material-ui/core', () => ({
  ListItem: 'ListItem',
  ListItemSecondaryAction: 'ListItemSecondaryAction',
  ListItemText: 'ListItemText',
  withStyles: () => component => component,
}));

4. Run tests again and record speed.

Expected behavior

While I would expect some overhead for module resolution. I would not expect such a drastic difference in speeds. If this is expected is it documented somewhere?

Link to repl or repo (highly encouraged)

I've tried to reproduce this in repl.it with no success. Unfortunately the project I'm experiencing this on is private.

I will try to create a new CRA 2.0 project and see if I can reproduce and publish here when I get a chance.

Run npx envinfo --preset jest

Paste the results here:

  System:
    OS: macOS 10.14.2
    CPU: (8) x64 Intel(R) Core(TM) i7-6820HQ CPU @ 2.70GHz
  Binaries:
    Node: 8.11.3 - ~/.nvm/versions/node/v8.11.3/bin/node
    Yarn: 1.12.3 - /usr/local/bin/yarn
    npm: 6.4.1 - ~/.nvm/versions/node/v8.11.3/bin/npm

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