How to Remove query strings from static resources

I launched a new business https://www.massagechairijoy.com/ 2 month ago. Recently, I found out my website speed is very low by using pindom checking where My site is at E level performance. I know that is too bad, I started to to find some answer from forum, and also try to install some plugin which can help it remove query strings. But nothing is changed.
Anyone can help me which plugin should be best in my situation or solution for it? Problem from theme?