One single component of web pages is consistently responsible for the majority of the latency on pages across the web: images.

Most of the common image formats on the web today were established over a decade ago and are based on technology from around that time. Some engineers at Google decided to figure out if there was a way to further compress lossy images like JPEG to make them load faster, while still preserving quality and resolution. As part of this effort, Google is releasing a developer preview of a new image format, WebP, that promises to significantly reduce the byte size of photos on the web, allowing web sites to load faster than before

Google:
Images and photos make up about 65% of the bytes transmitted per web page today. They can significantly slow down a user