GTMetrix changed its testing system, and all the sites that had A, B scores get C, E or F. Try to test your site here – https://tools.pingdom.com/
Every additional element on the page increases the dom structure. To reduce it remove the elements, there is no other way, unfortunately (for example limit the number of products in the slider, or remove some elements etc).
Also, read this article – https://xstore.helpscoutdocs.com/article/100-how-to-speed-up-my-website
