Engineers Paddy and Eoin from Tines encountered a perplexing issue where their front-end build times drastically increased due to a CSS-in-JS library called Linaria. After extensive debugging with tools like git bisect and Node DevTools, they discovered that the slowdown was related to how the `getDependencies` function in Linaria interacted with Webpack 5, particularly with the `Array.concat` method. The issue stemmed from Webpack 5's use of `Symbol.IsConcatSpreadable`, which forced `concat` to take a slow execution path if this symbol was set anywhere in the application. This problem was exacerbated by their implementation of Dark Mode, increasing the dependency graph size. They eventually refactored the function to avoid using `concat`, leading to significant performance improvements. The root cause was identified as a Webpack 5 feature intended for backward compatibility with Webpack 4. A fix was incorporated in Webpack 5.62.0, introducing an experimental `backCompat` option, further reducing build times.