With this option, App Center runs the test as part of the build operation, which adds time to the build.
While App Center Test validates that your app is ready to run on real devices, several time-consuming tasks run such as signing, checking permissions, etc. Next, the test must wait for a compatible device to become available, execute the test, and finally copy test logs, screenshots, etc. into the cloud.

Expect an additional 10 minutes of build time for this option.