Page speed depends on many factors such as server resource, amount of site images, amount of site visitor, external assets and etc.
In this article, we will describe how to quickly improve your site speed with some lightweight and effectively plugins.
First you need to settings your site speed from Theme Options -> Speed & Performance.
When you changing a theme, Maybe your images are not being displayed incorrect size and the full image will be loaded. This will increase the site size and also reduce the time load.
For further information, Refer to: Images are not displaying consistent in size.
Cache plugin will improve your website performance and help your site reduce the response time. We recomended plugin.
