Everyone hates slow websites and also decrease the user rating and ranking in search engine. You need to make sure that the CSS, JS and all images on the theme are optimized for the web. Your theme should be responsive and attractive to visitor’s eyes, but it should be a light weight so that website can load very fast on visitor’s browsers. While developing wordpress you keep few things in mind to speed up wordpress site.

·       Minify HTML, CSS, and Javascript

·       Gzip and compress components

·       Put stylesheets at the top

·       Put scripts at the bottom

·       Utilize browser caching

Also you can Find many plugin from wordpress.org e.g like w3 cache plugin and wp minify and many more.