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Not only CPU but slow networks and mobile screen sizes too. These are all very helpful for quick sanity checks and reproducing races.

Lighthouse also emulates slow-ish web on a mobile device. Though I would take its results with a grain of salt: it can show near-perfect green scores for a website that takes 3 seconds to render

Unless you mean that the menu is hidden behind the gear icon, calling it a hidden feature is a stretch.

https://developer.chrome.com/docs/devtools/device-mode/#cpu

Sufficiently deep in menus that I suspect most people here on HN would consider themselves familiar with devtools, yet wouldn't have been aware of this option.

Also available in Firefox Developer Tools. Just click on `No Throttling` and there's a menu of network speed options.

similarly i often let throttling on.. very nice way to tame dopamine rush :)