Sentry's Performance Monitoring instrumentation was used to instrument Jest, their frontend test suite, in order to gain insights into the performance of individual tests. The team found that the slowest tests were larger page views with many subcomponents and components accessing data from their data stores, which suggested poor-performing tests rather than a CI problem. To address this, they started using the node debugger to profile the test and identified that the component was not being unmounted after each test, causing change detection in components that should have been destroyed. By adding a `componentWillUnmount` method to the class component being tested and manually unmounting it after every test, their overall total time cut nearly in half. The instrumentation helped them identify performance issues and make targeted improvements to their tests.