Running tests in parallel can consume more system resources and potentially hit governor limits more frequently. By running tests sequentially, you may be less likely to exceed these limits, which can lead to test failures and complications.

To disable parallel tests in Salesforce, you can follow these steps:

1. Log in to your Salesforce organization.
2. Click on the Setup gear icon in the top-right corner.
3. In the Quick Find box, type “Apex Test Execution” and select the “Apex Test Execution” option.
4. On the Apex Test Execution page, you will see the “Options…”
5. In the Test Options dialog box, deselect the “Disable Parallel Apex Testing” option.
6. Click on the “Save” button to apply the changes.

Here is a screenshot that illustrates this process:

By following these steps, you have successfully disabled parallel tests in Salesforce. Now, when running the tests, they will execute sequentially instead of running in parallel across classes.

Parallel test execution can provide significant time savings and speed up the testing process, especially for large test suites. Salesforce introduced parallel test execution to improve overall development efficiency. Therefore, you should carefully consider whether to disable parallel execution based on the specific needs and characteristics of your Salesforce environment and your test suite. It may be necessary in some cases but unnecessary in others.