04: add intro and outro

Author: kettanaito

exercises/04.performance/FINISHED.mdx

@@ -1,3 +1,7 @@
 # Performance
 
-Great job! :tada: Take a rest and let's continue.
+Great job! :tada: The final exercise block is now behind you.
+
+Now you are equipped with the tools and techniques to approach any performance degradation in your tests. You know how to profile tests and how to act based on that. You have a better understanding of how Vitest runs your tests and what defaults it has, as well as how opting out from those defaults might yield a huge performance improvement!
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+But you also know that none of those approaches are a solution to a problem. They simply change how Vitest runs your tests and you need to be mindful when introducing those changes.

exercises/04.performance/README.mdx

@@ -1,3 +1,9 @@
 # Performance
 
-There's nothing for you to read here but I will have a lot to say on this topic once we come to this exercise!
+The second most common complain about automated tests (after them being flaky, of course) is that they are _slow_. And while flakiness often has a more unpredictable nature, performance issues with tests are more similar to a snowball. They might be there from the start but you won't notice them while having ten tests. They will become painfully apparent once you have a hundred.
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+**I don't want you to be punished for improving the test coverage of your software**. Neither do I want you to be left in the dark when you stare at a 10-minute long test run on CI and begin to question the rationale of your life's choices.
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+This exercise block will teach you how to deal with performance degradation in tests. How to profile slow test runs, explore and understand the CPU and memory consumption of tests, and what common issues that consumption may indicate.
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+But not before we talk about how Vitest works. What processes it has, what each process is responsible for, why your test files are wrapped in workers, and many other details that have a direct impact on performance.