Cypress will automatically balance your spec files across the available machines in your CI provider. Cypress calculates which spec file to run based on the data collected from previous runs. This ensures that your spec files run as fast as possible, with no need for manual configuration.
As more and more tests are recorded to the Cloud, Cypress can better predict how long a given spec file will take to run. To prevent irrelevant data from affecting the duration prediction, Cypress doesn't use old historical run data regarding the spec file.
Spec duration history analysis
With a duration estimation for each spec file of a test run, Cypress can distribute spec files to available CI resources in descending order of spec run duration. In this manner, the most time-consuming specs start first which minimizes the overall test run duration.
Duration estimation is done separately for every browser the spec file is tested against. This is helpful since performance characteristics vary by browser, and therefore it is perfectly acceptable to see different duration estimates for each browser a spec file was tested against.