Device Concurrency defines the number of devices an account can use in parallel at any one time. Accounts with higher levels of Device Concurrency are typically able to complete their test runs faster (depending on the number of devices targeted) because their tests will execute on more devices simultaneously.
Let's step through a quick example.
I have a test suite for an Android app that takes 10 minutes to run on a single device. My quality team requires that we support 20 device configurations.
If my Test Cloud subscription allows for 1 concurrent device, my test suite will take about 200 minutes to complete, or 3 hours and 20 minutes. (10 minutes per device X 20 target devices).
If my Test Cloud subscription allows for 5 concurrent devices, my test suite will take about 40 minutes to complete for a time savings of 2 hours and 40 minutes.
If you've ever waited for a build to finish on a Friday afternoon, the value of increased device concurrency is immediately apparent.