Slow automated tests
Automated tests can be considered slow when programmers stop running the whole test suite on their local machine because it is too time consuming. Most of the time you use CI servers such as Jenkins, CircleCI, Github Actions to run your tests on an external machine instead of your own. When you have a test suite that runs for an hour then it’s not efficient to run it on your computer. Browser end-to-end tests for your web project can take a really long time to execute. Running tests on a CI server for an hour is also not efficient. You as a developer need a fast feedback loop to know if your software works fine. Automated tests should help you with that.
Split tests between many CI machines to save time
A way to save you time is to make CI build as fast as possible. When you have tests taking e.g. 1 hour to run then you could leverage your CI server config and setup parallel jobs (parallel CI machines/nodes). Each of the parallel jobs can run a chunk of the test suite.
You need to divide your tests between parallel CI machines. When you have a 60 minutes test suite you can run 20 parallel jobs where each job runs a small set of tests and this should save you time. In an optimal scenario you would run tests for 3 minutes per job.
How to make sure each job runs for 3 minutes? As a first step you can apply a simple solution. Sort all of your test files alphabetically and divide them by the number of parallel jobs. Each of your test files can have a different execution time depending on how many test cases you have per test file and how complex each test case is. But you can end up with test files divided in a suboptimal way, and this is problematic. The image below illustrates a suboptimal split of tests between parallel CI jobs where one job runs too many tests and ends up being a bottleneck.