Steve Souders said on High Performance Web Site,

80-90% of the end-user response time is spent on the frontend.

Start there.

Web performance mostly depends on the frontend. This means to make the web faster, we have to optimize the frontend. For example, it contains following factors:

Minimizing payload size is one of the easiest to apply. To reduce the number and payload size, we have to reduce HTML, CSS, and image files. Especially, the file size of images takes most bandwidth in most cases. So I will introduce you to some tools for image optimization.