Rerendering and recomputing too much (ever encountered these chains of map() and filter() in render()?), many API calls, huge icon sets, huge custom fonts, CSS frameworks and JS frameworks, huge dependency trees with many instance of the same lib running sometime in several versions, big bundles... This heaviness in the name of convenience and branding is not free, it forces people to ditch perfectly fine hardware, which has a high environmental cost... not directly paid by people funding the code).

One of my pet peeves is managed text inputs, where render() is called each time you type a character, for a whole component tree if you are not careful and you happen to pass the content to some parent for some reason. Typing a message in Mattermost on the PinePhone is painful for this reason, there's just no reason typing should be slow even on slow hardware but managed component is considered good practice.

Making people buy newer / powerful and making people wait and using too much energy should be considered bad practice.