The amount of time a user spends on a website or an online service is the most significant factor in reducing or increasing the website’s environmental footprint. In other words, the less time the user spends on the website or online service, the smaller the environmental footprint.
You must therefore try to minimize the number of screens, steps and unnecessary interactions while at the same time ensuring that screens are not overloaded and too complex, which in itself can be counter-productive.

Group small images (e.g. the website’s interface images) in one larger image called a sprite sheet
This process significantly reduces the number of HTTP requests. There are several free online services for generating sprite sheets (Project Fondue’s CSS Sprite Generator, CSSsprites.com and SpriteMe.org).