TYPO3 Core

Concatenating the JS and CSS files (esp. in BE) is very expensive (up to 6% of rendering time).

Even more expensive than checking for file modifications is are the string operations while figuring out, what to include (resolveBackPath, isValidPath etc.).

So to be precise: Of course, we do not concatenate it every time. It's only about figuring out, how the hash of the concatenated file is (this is based on the included file names and their modification times - and this is already expensive).