feat: fill out READMEs for 04-03 problem and solution

exercises/04.performance/03.problem.test-isolation/README.mdx

@@ -1,3 +1,14 @@
 # Test isolation
 
-...
+Vitest runs every test file in a separate worker, which enables parallel execution and speeds up tests by default. 
+
+Each worker has its own isolated environment, meaning that if one test file mutates a global variable, other test files won't be affected by this mutation. This test file-level isolation is a great feature for preventing test interference.
+
+### The Problem
+While test isolation provides benefits, **it comes with a performance cost.** 
+
+When you have hundreds or thousands of test files, the overhead of spawning a worker for each file accumulates and can significantly degrade performance.
+
+### Your Task
+
+Follow the instructions to disable test isolation and observe the performance difference it makes in projects with a large number of test files.

exercises/04.performance/03.solution.test-isolation/README.mdx

@@ -32,4 +32,24 @@ This also means that you can benefit from correctly written tests in more ways t
 
 ## Your task
 
-👨‍💼 ...
+Opt out of the default test isolation and observe how this change affects how your tests are ran.
+
+### Configuration Change
+
+To disable test isolation and prevent Vitest from spawning a worker for each test file:
+
+1. Open your `vitest.config.js` (or `vitest.config.ts`)
+2. Add or modify the test configuration:
+
+```javascript
+export default defineConfig({
+  test: {
+    globals: true,
+    isolate: false,
+  },
+})
+```
+
+Then run `npm test`. 
+
+What change did this make to your test run?